SINGLE CRYSTAL MICROWAVE PLASMA ASSISTED CHEMICAL VAPOR DIAMOND SYNTHESIS AT HIGH PRESSURES AND HIGH POWER DENSITIES By Jing Lu A DISSERTATION Submitted to Michigan State University In partial fulfillment of the requirements for the degree of Electrical Engineering – Doctor of Philosophy 2013 ABSTRACT SINGLE CRYSTAL MICROWAVE PLASMA ASSISTED CHEMICAL VAPOR DIAMOND SYNTHESIS AT HIGH PRESSURES AND HIGH POWER DENSITIES By Jing Lu The main objectives of this dissertation research were to evaluate the existing microwave plasma assisted chemical vapor deposition (MPACVD) Reactor B in the high pressure regime of 180-280 torr by experimentally investigating single crystal diamond (SCD) synthesis, and experimentally defining the safe, efficient and stable process window for high growth rate high quality SCD production. An experimental methodology was developed that determined the safe and efficient operating regime for Reactor B. This methodology first defined the nonlinear relationships between the input power, discharge average power density, pressure and substrate temperature; i.e. it established an operating field map at high pressure and power densities for Reactor B. Then a safe and efficient reactor operating variables space over the 180-280 torr pressure regime was identified within the operating field map. When operating within these conditions, discharge power density was increased to 100-600 W/cm 3 by increasing the operating pressure, reactor wall reactions were also minimized, and the safe, efficient and low maintenance operation of Reactor B was enabled over a wide range of operating conditions. For Reactor B the operating field map and the safe and efficient operating regime was specifically defined for SCD synthesis. While operating within a safe and efficient reactor operating window, SCD synthesis was experimentally demonstrated from 180 to 280 torr. At a constant pressure of 240 torr a high quality, high growth rate SCD substrate temperature window was experimentally identified between 1030 and 1250 ° In particular using C. feed gases with nitrogen impurity levels of less than 10 ppm SCD was synthesized with growth rates of 20-45 µm/h. The SCD synthesis experiments demonstrated that as the pressure and discharge absorbed power density increased the diamond deposition rate increased. Diamond synthesis rates and quality surpass those that were achieved by synthesizing SCD at lower pressure and with earlier reactor technologies. As pressure was increased the experimental variable window to grow high quality diamond also expanded and larger methane concentrations (5-7%) were able to synthesize high quality diamond. When nitrogen impurity levels were reduced below 10 ppm in the gas phase the quality of the synthesized diamond was of type IIa or better. After laser cutting and polishing high quality diamond plates were synthesized. These experiments demonstrated that MPACVD diamond synthesis rates and diamond quality increased and improved respectively as the operating pressure increased. Copyright by JING LU 2013 To H.O.T. Heejun, Woohyuk, Tony, Kangta and Jaewon, together, forever. v ACKNOWLEDGEMENTS I would like to express my sincere gratitude to my major advisor, Dr. Jes Asmussen, for his continual moral and financial support, constant encouragement and guidance, and editorial and technical suggestions during the course of this dissertation research. Sincere appreciation is also extended to Dr. Timothy A. Grotjohn, Dr. Donnie K. Reinhard and Dr. Greg M. Swain for serving on my advisory committee. Next I would like to thank Dr. Thomas Schuelke and the Fraunhofer USA Center for Coating and Laser Applications (CCL) personnel for providing assistance for diamond cutting, polishing, and characterization, especially Matthias Muehls for helping with the Birefringence and transmission measurements. Sincere appreciation for Dr. Xingyi Yang with the Raman spectroscopy instruments set up, Karl Dersch, Brian Wright, and Roxanne Peacock with their technical support. Additional thanks are given to fellow graduate students for their valuable assistance during the course of this work. Finally, special thanks to my family for their understanding, encouragement, and moral support in completing this degree. This work was supported by the Richard M. Hong Chaired Professorship, Fraunhofer USA CCL, and the Block-Gift Program of II-VI Foundation. vi TABLE OF CONTENTS LIST OF TABLES ............................................................................................ xi LIST OF FIGURES ........................................................................................xiii CHAPTER 1 ..................................................................................................... 1 INTRODUCTION ............................................................................................. 1 1.1 Research motivation ............................................................................ 1 1.2 Research objectives and approach...................................................... 4 1.3 Dissertation outline .............................................................................. 5 CHAPTER 2 ..................................................................................................... 7 MICROWAVE PLASMA ASSISTED CVD DIAMOND SYNTHESIS BACKGROUND ............................................................................................... 7 2.1 Introduction .......................................................................................... 7 2.2 Introduction of single crystal diamond synthesis by MPACVD ............. 8 2.2.1 Properties of diamond ................................................................ 8 2.2.2 CVD diamond synthesis process .............................................. 10 2.2.3 Substrate surface chemistry in diamond growth process ......... 11 2.2.4 Main radicals in diamond growth process................................. 14 2.3 Literature review of MPACVD single crystal diamond synthesis ........ 20 2.3.1 Carnegie Institution of Washington, U.S.A................................ 20 2.3.1.1 Large single crystal diamond fabrication with high growth-rate .................................................................................. 21 2.3.1.2 Enhanced mechanical properties .................................... 26 2.3.1.3 Production of high optical quality single crystal diamond 30 2.3.2 Diamond Research Center, AIST, Japan .................................. 34 2.3.2.1 High rate SCD growth with nitrogen addition ................... 34 2.3.2.2 Numerical and experimental studies of a new discharge reactor ......................................................................................... 39 2.3.2.3 Synthesis of large single crystal diamond plates by lift-off process ....................................................................................... 45 2.3.3 LIMHP/CNRS, Paris, France .................................................... 52 2.3.3.1 Plasma modeling of CVD reactors .................................. 52 2.3.3.2 High quality synthesized SCD growth ............................. 54 2.3.3.3 Multiple growth sectors on the substrate surface ............ 58 2.3.4 Institute of Applied Physics RAS, Nizhny Novgorod, Russia .... 62 2.3.4.1 Single crystal diamond growth ........................................ 62 2.3.4.2 Single crystal diamond growth at continuous and pulsed mode ........................................................................................... 65 2.3.5 University of Bristol, UK ............................................................ 69 vii 2.3.6 Element Six Ltd., UK ................................................................ 77 2.3.7 Summary .................................................................................. 78 CHAPTER 3 ................................................................................................... 81 THE MPACVD REACTOR AND ASSOCIATED SYSTEMS ........................... 81 3.1 Introduction ........................................................................................ 81 3.2 Summary of previous work ................................................................ 82 3.2.1 Reactor A for polycrystalline diamond deposition ..................... 84 3.2.2 Reactor B for polycrystalline diamond deposition ..................... 91 3.2.3 Reactor B for single crystal diamond synthesis ........................ 96 3.3 The microwave plasma assisted chemical vapor deposition experimental system ................................................................................ 97 3.3.1 Introduction of microwave plasma reactor ................................ 98 3.3.2 Other subsystems of the MPACVD experimental system ....... 102 3.3.2.1 Microwave power supply and circuit transmission network .................................................................................................. 102 3.3.2.2 Gas flow control ............................................................ 103 3.3.2.3 Vacuum pumping and pressure control ......................... 104 3.3.2.4 Additional components .................................................. 105 3.4 The cooling stage and substrate holder configurations .................... 106 3.4.1 Cooling stage ......................................................................... 106 3.4.2 Substrate holder ..................................................................... 108 3.5 External microwave coupling system and microwave coupling efficiencies ............................................................................................. 113 CHAPTER 4 ................................................................................................. 118 THE MPACVD REACTOR OPERATION AND DIAMOND SYNTHESIS PROCEDURES............................................................................................ 118 4.1 Introduction ...................................................................................... 118 4.2 Substrates for diamond deposition .................................................. 119 4.2.1 Substrates for polycrystalline diamond deposition .................. 119 4.2.2 Substrates for single crystal diamond deposition ................... 120 4.2.3 Single crystal diamond pre-deposition cleaning procedure .... 124 4.2.4 Single crystal diamond post-deposition cleaning procedure ... 125 4.3 The MPACVD experimental system operational procedures ........... 127 4.3.1 Experiment start up procedure ............................................... 127 4.3.2 Experiment shut down procedure ........................................... 129 4.3.3 MPACVD experimental system cleaning procedure ............... 130 4.4 Evaluation of the synthesized single crystal diamond ...................... 133 4.4.1 Diamond growth rate calculation ............................................ 133 4.4.2 Diamond quality characterization ........................................... 134 4.4.2.1 Single crystal diamond surface morphology .................. 135 4.4.2.2 Raman spectroscopy..................................................... 135 4.4.2.3 Secondary ion mass spectrometry analysis .................. 138 viii 4.4.2.4 Optical transmission measurements ............................. 141 4.4.2.5 Birefringence imaging.................................................... 143 4.5 The multi-dimensional experimental variable space ........................ 145 CHAPTER 5 ................................................................................................. 148 EVALUATION OF MPACVD REACTOR B’S PERFORMANCE AT HIGH PRESSURES ............................................................................................... 148 5.1 Introduction ...................................................................................... 148 5.2 Experimental techniques ................................................................. 149 5.2.1 The experimental variables that determine the operating field map ................................................................................................. 149 5.2.2 The data acquisition in field map measurement ..................... 153 5.2.2.1 Substrate temperature measurement ............................ 154 5.2.2.2 Photography of the reactor microwave discharge ......... 156 5.2.2.3 Determination of microwave power density ................... 162 5.3 Experimental Reactor operating field map ....................................... 165 5.4 High pressure discharge behavior ................................................... 174 CHAPTER 6 ................................................................................................. 178 PROCESS CONTROL AND MATCHING METHODS FOR SINGLE CRYSTAL DIAMOND SYNTHESIS ............................................................................... 178 6.1 Introduction ...................................................................................... 178 6.2 MPACVD reactor operational principles ........................................... 180 6.3 Discharge formation, position control and reactor matching strategy .............................................................................................................. 183 6.4 Reactor matching: tuning via variation of Ls .................................... 189 6.5 Substrate temperature control versus time during the SCD synthesis process .................................................................................................. 196 6.5.1 Determination of SCD substrate temperature ......................... 197 6.5.2 Controlling the substrate temperature versus time ................. 200 6.5.3 Controlling the substrate temperature via variation of substrate holder design ................................................................................... 207 6.6 Process control via substrate position variation ............................... 209 CHAPTER 7 ................................................................................................. 212 MICROWAVE PLASMA ASSISTED CVD SINGLE CRYSTAL DIAMOND SYNTHESIS: EXPERIMENTAL RESULTS .................................................. 212 7.1 Introduction ...................................................................................... 212 7.2 The experimental variable space for single crystal diamond synthesis .............................................................................................................. 214 7.3 Hydrogen plasma etching of the HPHT single crystal diamond substrates .............................................................................................. 222 7.3.1 Hydrogen plasma etching experimental techniques ............... 223 7.3.2 Diamond etching rate versus etching temperature ................. 224 ix 7.3.3 Description of the hydrogen etched substrate surface ........... 226 7.3.4 Summary ................................................................................ 233 7.4 Experimental evaluation of single crystal diamond synthesis in multi-variable space ............................................................................... 235 7.4.1 Diamond growth versus the variation of substrate position .... 236 7.4.2 Diamond growth with nitrogen addition................................... 243 7.4.3 Diamond growth rate versus simplified multi-variable space .. 253 7.4.3.1 Introduction ................................................................... 253 7.4.3.2 Growth rate versus substrate temperature .................... 255 7.4.3.3 Growth rate versus pressure ......................................... 260 7.4.4 Diamond morphology versus simplified multi-variable space . 267 7.4.5 Diamond quality characterization ........................................... 279 7.4.5.1 High quality high growth rate SCD synthesis window ... 279 7.4.5.2 Characterization of diamond plates ............................... 284 7.4.6 Summary ................................................................................ 289 7.5 Synthesis of large area single crystal diamond plates ..................... 293 7.5.1 Introduction ............................................................................. 293 7.5.2 Approach of large size single crystal diamond plates synthesis ........................................................................................................ 293 7.5.3 Initial results of large area single crystal diamond substrate synthesis ......................................................................................... 297 7.5.3.1 Single crystal diamond synthesis on sides of HPHT diamond seed............................................................................ 297 7.5.3.2 Long-time single crystal diamond synthesis process .... 299 7.5.3.3 Synthesis resumption after interruption ......................... 302 CHAPTER 8 ................................................................................................. 304 SUMMARY AND RECOMMENTDATION..................................................... 304 8.1 Summary ......................................................................................... 304 8.2 Publications resulting from this dissertation research ...................... 309 8.3 A Comparison of the experimental data from this dissertation with the recent work of others ............................................................................. 309 8.4 Recommendations for future research............................................. 311 APPENDICES .............................................................................................. 313 APPENDIX A Configurations of Substrate Holders ............................. 314 APPENDIX B Calculation of Nitrogen Impurity Concentration ............ 316 APPENDIX C Experimental Data for Diamond Etching ....................... 318 APPENDIX D Experimental Data for Diamond Synthesis ................... 320 BIBLIOGRAPHY .......................................................................................... 330 x LIST OF TABLES Table 2.1 – Diamond properties…….……………………………………………...9 Table 2.2 – A set of reactions between substrate and gas mixture [11]….…...12 3 Table 2.3 – A reduced methyl diamond growth mechanism. Units for k: cm , mo, s [10]……………………………………...……………………...15 Table 2.4 – Growth parameters for SCD specimens shown in Figure 2.4 [12]...............................................................................................24 Table 2.5 – Experimental conditions of recent MPACVD SCD synthesis from various research groups [13, 30-39, 43-48, 58-60] ………….....79 Table 4.1 – Specifications of the HPHT diamond seeds from Sumitomo [84]………………………………………………………...…………122 Table 5.1 – Fixed experimental variables in the reactor operating field map measurement…………………………………………...……….....151 Table 5.2 – The variable experimental variables in the reactor operating field map measurement………………………………………..………152 Table 5.3 – Camera settings for discharge photograph…………………….…157 Table 7.1 – Fixed input experimental variables for the SCD synthesis experiments...............................................................................216 Table 7.2 – The variable input experimental variables for the SCD synthesis experiments…………………………………………….…………...218 Table 7.3 – Experimental variables used in the hydrogen only plasma etching experiments…………………………………….…………………...224 Table 7.4 – Results of hydrogen plasma etching experiments………….…..234 Table 7.5 – Experimental variables for high quality high growth rate SCD synthesis process…….…………………………………………….292 Table A.1 – Configurations of substrate holders for single crystal diamond xi synthesis experiments. The schematic drawings of holders are shown in Figures 3.17-3.20 and A.1……………….……… …….314 Table C.1 – Fixed experimental variables used in the hydrogen only plasma etching experiments……………………………….……………….318 Table C.2 – Experimental data for single crystal diamond etching experiments...............................................................................318 Table D.1 – Fixed experimental variables for the single crystal diamond synthesis experiments………………………………….………….320 Table D.2 – The flow rates of methane employed in single crystal diamond synthesis experiments for various concentrations. ……….……321 Table D.3 – The flow rates of nitrogen employed in single crystal diamond synthesis experiments for various concentrations………….…..321 Table D.4 – The associated positions of short plate (Ls) for varied L2 employed in single crystal diamond synthesis experiments……….……....321 Table D.5 – Experimental data for single crystal diamond synthesis experiments. The information about other variables such as L1, Ls and feed gases flow rates can be looked up in Tables A.1, and D.1-D.4……………………………………………………………....322 xii LIST OF FIGURES Figure 2.1 – General MPACVD diamond growth process. For interpretation of the references to color in this and all other figures, the reader is referred to the electronic version of this dissertation......………..11 Figure 2.2 – Examples of reaction sequences leading to growth on the (a) {110} and (b) {111} faces of diamond. The text below the images indicates the reactions according to the labeling in Table 2.2. Dark gray circles are diamond, light gray circles are C atoms in chemisorbed hydrocarbons, and white circles are H atoms [11]…………………………………………………………………….13 Figure 2.3 – Process map in [CH3]-[H] space showing the operating ranges of the main CVD diamond growth processes [10]. The evolution of the operation point of the MPACVD reactor from LIMHP as a function of working pressure and methane concentration has been added [4]…………………………………………..…..………18 Figure 2.4 – Selected near colorless and colorless SCD specimens, clockwise from the top: (1) SCD-1: light brown, brilliant cut and polished single crystal containing nitrogen (~0.5 carat); (2) SCD-2: near colorless, 0.2 carat brilliant cut and polished single crystal produced from a ~1 carat block; (3) SCD-3: colorless 1.4 carat bullet shape single crystal produced from a ~2.2 carat block [3]…………………………………………………….…….………….23 Figure 2.5 – Photoluminescence (PL) spectra of natural IIa diamond, light brown, and colorless synthesized SCD (300 K) [3]…………..….25 Figure 2.6 – UV-visible absorption spectra of natural type IIa diamond, nitrogen doped light brown and colorless synthesized SCD [3]…………..26 Figure 2.7 – Vickers hardness and fracture toughness on [100] faces of various diamonds in the {100} directions [16]…..…………….….29 Figure 2.8 – UV-visible absorption spectra of synthesized SCD (300 K). a) before annealing, b) after annealed at 1800 ° for 2 min. The C inset shows several examples of annealed diamond plates [17]..............................................................................................31 xiii Figure 2.9 – The picture on the right shows a 2.4 carat synthesized SCD compared with 0.25 carat synthesized SCD. The pictures on the left is an example of the evolution of synthesized SCD sample (A to B to C to D) starting with 13.5 carat crystal block (A) to the 2.3 carat cut gem anvil (D) [13]………………………………………...33 Figure 2.10 – Plot on the left: Effect of reactor pressure on the growth rate for open and enclosed type holders [22]. Figures on the right: schematic illustration of “open type” and “enclosed type” substrate holders [21]………………………………………………………….36 Figure 2.11 – Effect of growth temperature on the growth rate for open type holder. Optical microscope image shows CVD diamond with the thickness of about 100 µm grown on a substrate. Growth temperature corresponding to each image is indicated in the plot [22]……………………………………………………..……………..37 Figure 2.12 – Large CVD diamonds grown by multi-step high growth rate synthesis process. (a) A 10 mm thick, 4.7 ct CVD diamond, (b) a 9.6 mm thick, 3.5 ct CVD diamond, (c) an 8.7 mm thick, 4.4 ct CVD diamond [22]………………………………………………….38 Figure 2.13 – Schematic illustration of the cross section of the reactor designs: (a) conventional configuration, (b) new configuration [28]…..…40 Figure 2.14 – Contours of electron number densities for the conventional configuration. Gas pressures are set to (a) 80 torr, (b) 110 torr, and (c) 150 torr, respectively [28]…………………………….…..41 Figure 2.15 – Contours of electron number densities of for the new configuration. Gas pressures are set to (a) 80 torr, (b) 110 torr, and (c) 150 torr, respectively [28]..……………………….………42 Figure 2.16 – Photograph of the discharge region in the horizontal direction [27]…………………………………………………..………….…….43 Figure 2.17 – Radial distribution of the growth rate (open squares; left vertical axis) that was experimentally obtained and the power density (solid line with solid circles; right vertical axis) that was numerically obtained. The horizontal axis represents the radial coordinate [27]..………………………………………………..…..44 Figure 2.18 – Lift-off process with ion implantation [36]……………………….46 xiv Figure 2.19 – A 10 mm x 10 mm x 0.4 mm synthesized SCD plate (right) separated from a HPHT diamond substrate (left) by the combination of high rate growth and lift-off process using ion implantation [30]. …………………………………………………..46 Figure 2.20 – Schematic illustration of a crystal enlarging process by combination of lift-off process and side-surface growth [32].....47 Figure 2.21 – A half-inch single-crystal diamond produced by the step 4 of process shown in Figure 2.20. Arrows correspond to the growth direction [32]………………………………………………………..48 Figure 2.22 – (a) Photograph of a half-inch substrate plate by cutting and polishing the diamond in Figure 2.21. (b) Photograph of a half-inch SCD plate produced with N2 addition from the substrate plate by lift-off process using ion implantation. (c) Polarized light microscope image of the half-inch single-crystal CVD diamond plate [32]……………………………………………………….…….49 Figure 2.23 – Procedure to produce clones and tiled clones [37]……………..50 Figure 2.24 – Photograph of a free-standing SCD plate with size of around 20 mm x 20 mm [37]……………………………………………….….51 Figure 2.25 – MPACVD reactors and their electric field distribution. Light red co lo r in d ica t e s h igh e r e le ct ric f ie ld . (a ) Ho m e -m a d e metallic-chamber reactor; (b) bell-jar reactor co-developed between LIMHP and Plassys company, showed opened on the picture [57]……………………………………………………...…..53 Figure 2.26 – (a) Diamond growth rate as a function of nitrogen addition in the 3 gas phase (CH4/H2 = 4%, MWPD = 95 W/cm ). Optical and PL images obtained under UV light for an undoped freestanding CVD diamond crystal and a 2 ppm N2 doped. (b) Evolution of the growth rate as a function of microwave power density at a constant surface temperature (850 ° C) and for two different methane concentrations [57]……………………………..………56 Figure 2.27 – Photoluminescence spectra obtained at 77 K using a 514 nm green laser for excitation. The corresponding diamond layers xv were grown with 100 ppm N2 at (a) 950 ° (b) 875 ° and (c) C, C 800 ° For clarity, curves have been vertically shifted [46]..…57 C. Figure 2.28 – Optical images of the free-standing synthesized SCD samples obtained after laser cutting and polishing, and grown with (1) no intentional N2 addition, (2) 2 ppm of N2, (3) 4 ppm of N2, (4) and (5) 6 ppm of N2, (6) 10 ppm of N2 in the gas phase [45]……...…58 Figure 2.29 – Depiction of the {100}, {111}, {110}, and {113} growth sectors (bottom row) in HPHT diamond substrates, which are cut from an initial HPHT crystal with multiple sectors (top row) [52]. …..60 Figure 2.30 – Highlighting the growth sectors of HPHT substrates by: a) H2/O2 plasma etching pre-treatment; b) DiamondView imaging, and c) scanning electron microscopy with a large probe current (>1 nA). The d) panel lables the identified growth sectors (same colour scheme as in Figure 2.29) [52]………………………………..….60 Figure 2.31 – Experimental setup: (1) cylindrical cavity, (2) coaxial waveguide, (3) rectangular waveguide, (4) circulator with a match load microwave to absorb reflected microwave power, (5) magnetron, (6) microwave discharge, (7) quartz cell, (8) buffer vacuum volume, (9) pump out system, (10) gas-feed system, (11) magnetron power supply, (12) control PC, (13) diagnostic window, (14) SOLAR MS 3504 monochromator, (15) photomultiplier, (16) digital oscilloscope, (17) PC, (18) monochromator controller, (19) flat substrate holder, and (20) holder in the shape of truncated cone (trapezoid holder) [59]........................................................63 Figure 2.32 – Dependence of the MWPD on the gas pressure for two different configurations of the substrate holder. Letters A, B, C, and D (circles) mark the parameters, for which the comparison of the diamond deposition was made [59]………………………….…..64 Figure 2.33 – Photos of the samples after the deposition process [59]……....65 Figure 2.34 – Dependence of the MWPD on the gas pressure: solid lines – CW mode and dash lines – PW mode [61]……………………...……66 xvi Figure 2.35 – Dependence of the SCD growth rate on the gas pressure for CW and PW regimes at the same MWPD of 200 W/cm 3 and the methane content of 4% (circle) and 8% (triangle). Letters on the graph corresponds to letters in Figure 2.34 [61]………………..67 Figure 2.36 – Schematic diagram of the MPACVD reactor illustrating the position of the substrate, plasma ball, and the side arms for probing by CRDS [106]……………………………………………70 Figure 2.37 – 2D (r, z) plots of the calculated (left) electron and (right) H atom concentration, for substrate holder diameter = 3 cm and input power = 1.5 kW. The scales on the left and right under the plot belong to two plots of electron and hydrogen atom concentration, respectively [107]……………………………………………..……..71 Figure 2.38 – 2D (r, z) plots of the calculated (left) gas temperature, T gas, in Kelvin and (right) CH 3 number density, for substrate holder diameter = 3 cm and input power = 1.5 kW. The scales under the plot belong to two plots of concentration, respectively [107]…..73 Figure 2.39 – Filled symbols: column densities for C2 and CH radicals (left hand scale) and for H atoms (right hand scale) plotted as functions of (a) CH4 flow rate, (b) Ar flow rate, (c) applied MW power P, and (d) total pressure p. The open symbols show values for the corresponding quantities returned by the 2D model calcula tions [106]…………………………………… ..……75 Figure 3.1 – (a) Cross sectional view of the Reactor A, (b) the modified substrate holder with reduced inner conductor (cooling stage) radius of Reactor B. The z = 0 plane separates the cylindrical and coaxial sections of the reactor [68]. The other dimensions of Reactor B are the same as Reactor A…………………………….83 Figure 3.2 – The growth rate (total weight gain and linear) versus methane concentration [66]. (Experimental condition: 135 torr, 600 sccm, 3 1058 ° 5 h, 22.5 W/cm )………………………………………….86 C, Figure 3.3 – The growth rate (total weight gain and linear) versus substrate temperature [66]. (Experimental condition: 135 torr, 618 sccm, 3%, xvii 3 10 h, 21.4 W/cm )…………………………………………………...86 Figure 3.4 – Operating field map for Reactor A, i.e. substrate center temperature versus pressure and absorbed microwave power for the deposition plasma [67]……………………………………….…88 Figure 3.5 – A free-standing polycrystalline diamond sample, 75 µm thick [67]…………………………………………………………………….90 Figure 3.6 – A lapped and polished polycrystalline diamond sample, 35 µm thick, mounted for an ion beam electron stripping application. The window opening is 1 cm x 1 cm [67]…………………………….…90 Figure 3.7 – Substrate temperature versus absorbed microwave power at various operating pressures [68]………………………………..…92 Figure 3.8 – (a) substrate temperature and (b) diamond growth rate versus substrate position [68]……………………………………………....93 Figure 3.9 – Diamond growth rate with increasing operating pressure with methane concentration ranging from 2 to 5% with no addition of nitrogen gas into the system [68]……………………………….….94 Figure 3.10 – An example of free-standing diamond film adjacent to a quarter dollar coin, after being polished, lapped and Si substrate removal via chemical etching [68]……………………………………....….95 Figure 3.11 – Overall setup of MPACVD Reactor B experimental system……98 Figure 3.12 – Cross section of microwave plasma Reactor B…………….......99 Figure 3.13 – Detailed cross section of the inner conductor water cooling stage, substrate holder, and shims configurations. Units are in inches [1]………………………………………………..…………………..101 Figure 3.14 – Microwave power supply and waveguide network subsystem [1]………………………………………………..………………..…103 Figure 3.15 – Drawing of cooling stage inner conductor cross sections. Units are in inches [1]………………………………..………………..….107 Figure 3.16 – Configuration of quartz tube…………………………………….108 xviii Figure 3.17 – Drawings of “one-piece” substrate holder for SCD synthesis. Units are in inches [1]………………………………………….....110 Figure 3.18 – Drawings of substrate holder for polycrystalline diamond deposition (or bottom piece of “two-piece” substrate holder for SCD deposition). Units are in inches [1]………………………..111 Figure 3.19 – 3D top view of the top piece of “two-piece” substrate holder for SCD synthesis………………………………………………….…112 Figure 3.20 – Drawings of the top piece of “two-piece” substrate holder for SCD synthesis (Side view and top view, units in mm)…….…..112 Figure 3.21 – External microwave system employed for MPACVD diamond synthesis [79]…………………………………………………..….114 Figure 3.22 – The external microwave system used with the MPACVD. Note that reactor matching takes place at the input plane of the cavity applicator which is just one to one and a half wavelengths away from the discharge [79]…………………………………………..116 Figure 4.1 – The illustration of the shape of a HPHT diamond seed from Sumitomo [84]……………………………………………………...121 Figure 4.2 – Optical micrograph of the side view of a HPHT seed (25x magnification)............................................................................123 Figure 4.3 – Optical micrograph of the top view of a HPHT diamond seed (25x magnification)………………………………………………………123 Figure 4.4 – Example Raman spectra of a SCD sample synthesized by Reactor B (Sample 80). The inset is the close up of the peak of the spectra. (Experimental condition: 240 torr, 6% CH4/H2)…..137 Figure 4.5 – Schematic diagram of a SIMS analysis system [86]…………...140 Figure 4.6 – Depth profiling of the concentration of N and Si in SCD sample 38 (Experimental condition: 240 torr, 5% CH 4 /H 2 , 200 ppm N2/H2).......................................................................................140 Figure 4.7 – Schematic diagram of the spectrophotometer…………………..142 xix Figure 4.8 – Birefringence image of an HPHT diamond seed, 50 ms exposure time [88]……………………………………………………………..144 Figure 4.9 – Block diagram of the experimental variable groups…………....147 Figure 5.1 – Block diagram of the experimental variables that were measured when determining the reactor operating field map……………..150 Figure 5.2 – Examples of discharge photographs: (a) p = 180 torr, Pabs = 2.636 kW, exposure time = 1/2000 sec; (a) p = 240 torr, Pabs = 1.964 kW, exposure time = 1/2000 sec; (c) p = 240 torr, Pabs = 2.466 kW, exposure time = 1/4000 sec. (d) p = 240 torr, Pabs = 2.753 kW, exposure time = 1/4000 sec……………………………………....159 Figure 5.3 – Examples of discharge photographs taken under the same condition: pressure = 240 torr, Pabs = 2.115 kW, exposure time = 1/4000 sec………………………………………………………….161 Figure 5.4 – Example of discharge photograph with scale for power density calculation. Pressure = 180 torr, Pabs = 2.028 kW, exposure time = 1/2000 sec………………………………………………………..163 Figure 5.5 – Examples of the operating field map curves along with the associated discharge photographs for 60 torr and 240 torr for Reactor B. As shown in the photo inserts, the discharge was hovering over a 2.54 cm diameter silicon wafer…………….…..167 Figure 5.6 – Operating field map and the corresponding power density curves for Reactor B…………………………………………………….…170 Figure 5.7 – Operating field map curves and the identification of the efficient and safe experimental diamond synthesis region for Reactor B................................................................................................173 Figure 5.8 – Photographs of the discharge over the silicon substrate as the pressure was increased from 60 torr to 240 torr. The microwave absorbed power ranged from 1.331 kW to 2.087 kW as xx indicated…………………………………………………………….174 Figure 5.9 – Power density versus pressure for Reactors A, B and C. The data of Reactors A and C are from Ref [90]……………………….…..176 Figure 6.1 – Block diagram of experimental variables in microwave plasma controlling process. ……………………………………………….185 Figure 6.2 – Variation of the Pref/Pinc versus absorbed power versus different Ls positions. The curve on the top is the field map curve for substrate temperature (left axis) versus absorbed power when Ls is held fixed at 21.6 cm. Experimental conditions: 2.54 cm diameter silicon substrate, p = 180 torr, CH4/H2 = 3%, Lp = 3.56 cm, L1 = 52.87 mm, L2 = 58.6 mm, Zs = -5.73 mm…………….191 Figure 6.3 – Substrate temperature and Pref/Pinc versus absorbed power at pressures 120 torr, 180 torr and 240 torr when the reactor was operated in a well matched. Experimental conditions: 2.54 cm diameter silicon substrate, CH4/H2 = 3%, Lp = 3.56 cm, Ls = 21.6 cm, L1 = 52.87 mm, L2 = 58.6 mm, Zs = -5.73 mm………….....194 Figure 6.4 – An example of linear fit of substrate temperature measurements in an 8 hour SCD synthesis experiment…………………………....199 Figure 6.5 – Variation of the operating field map, i.e. from curve 1 to curve 2, versus synthesis time, t……………………………………………201 Figure 6.6 – Controlling the input power versus time to achieve constant substrate temperature versus time. The temperature was held constant at the initial temperature by moving the reactor operating point from A to C as the input power was reduced……………..203 Figure 6.7 – Example of substrate temperature adjustment by absorbed power variation during SCD deposition experiment. The circled data represents the substrate temperature variation when the absorbed power was changed…………………………………....205 xxi Figure 6.8 – SCD synthesis process control via varying the pressure from p1 to p2. Curve 3 represents the operating field map curve at t =t1 and a pressure of p2……………………………………………………...206 Figure 6.9 – Variation of absorbed power density and substrate temperature versus substrate position for Reactor B by polycrystalline diamond deposition [1]. Experimental conditions: p = 240 torr, CH4/H2 = 3%, L1 = 5.65 cm……………………………………………………..…211 Figure 7.1 – Block diagram of experimental variables in SCD synthesis experiments………………………………………………………...215 Figure 7.2 – Etching rate by linear encoder versus substrate temperature in hydrogen plasma etching experiments. (Experimental conditions: 240 torr)……………………………………………………………..226 Figure 7.3 – Examples of surface morphologies from samples which were etched with low, medium, and high etching temperature. The pictures on first row are the top view of the substrate surface. The pictures in the second row are “zoomed in” photos from a portion of the substrate surface as indicated by the square located on the substrate surface in the photos on the first row. The pictures on third row are side views of the substrate surface……………….228 Figure 7.4 – Examples of measurement of size of etch pits: (a) top view of sample 44; (b) side view of the deposition edge of sample 45..............................................................................................229 Figure 7.5 – Examples of revealed growth sectors in HPHT diamond substrates. Usually the bottom surface of the substrate samples revealed the various growth sectors. However sample 45, which is shown enclosed above, is reversed. It was the only sample found to exhibit this reverse behavior…………………………………...232 Figure 7.6 – The numerical simulation results of variation of electric field inside the reactor versus Zs for Reactor B (no discharge)…….…..….239 Figure 7.7 – Growth rate and substrate temperature versus Zs for Reactor B. xxii Experimental conditions: pressure = 240 torr, CH4/H2 = 5%, L1 = 52.66 cm, input power = 2.2 kW………………………………….240 Figure 7.8 – Diamond growth rate versus additional nitrogen concentration in the gas phase for Reactor A and B. Experimental conditions for Reactor B: CH4/H2 = 5%, Zs = -7.54 mm. The experimental data for reactor A was from ref. [101]…………………………………..245 Figure 7.9 – Growth rate and nitrogen content in crystal versus total nitrogen concentration in the gas phase. Experimental conditions: pressure = 240 torr, CH4/H2 = 5%, Zs = -5.73 mm………………………...248 Figure 7.10 – Raman FWHM and nitrogen content in the crystal versus total nitrogen concentration in the gas phase. Experimental conditions: pressure = 240 torr, CH4/H2 = 5%, Zs = -5.73 mm……….…..249 Figure 7.11 – Micrographs of the surfaces samples grown with different additional nitrogen concentrations and substrate temperatures. Experimental conditions: pressure = 240 torr, CH4/H2 = 5%, Zs = -5.73 mm. Rtm is the average distances between the surface peaks and valleys……………………………………….…….....252 Figure 7.12 – Block diagram of experimental variables simplified for SCD synthesis process………………………………………….….....254 Figure 7.13 – SCD linear growth rate versus substrate temperature for Reactor B. Experimental conditions: pressure = 240 torr, CH4/H2 = 6%, L2 = 58.6 mm, Ls = 21.6 cm, Zs = -3.34 mm – -5.73 mm…....256 Figure 7.14 – SCD linear growth rate versus substrate temperature for different methane concentrations. Experimental conditions: pressure = 240 torr, CH4/H2 = 4-7%, L2 = 58.6 mm, Ls = 21.6 cm, Zs = -3.34 mm – -5.73 mm………………………………………………………...258 Figure 7.15 – Growth rate versus pressure for different methane xxiii concentrations for Reactor B. Experimental conditions: pressure = 180-280 torr, CH4/H2 = 5-6%, Ls = 21.6 cm, L1 = 54.11 mm, L2 = 58.6 mm, Zs = -4.49 mm…………………………………..…..262 Figure 7.16 – Growth rate versus microwave power density for different methane concentration for Reactor B. Experimental condition: pressure = 180-280 torr, CH4/H2 = 5-6%, Ls = 21.6 cm, L1 = 54.11 mm, L2 = 58.6 mm, Zs = -4.49 mm………………………...263 Figure 7.17 – Shifting of the curves of growth rate versus substrate temperature as pressure varies………………………………....265 Figure 7.18 – Micrographs of the deposited diamond surfaces versus substrate temperature for the samples from the experiments presented in Figure 7.13. Experimental conditions: pressure = 240 torr, CH4/H2 = 6%, L2 = 58.6 mm, Ls = 21.6 cm, Zs = -3.34 mm – -5.73 mm. Rtm is the average distances between the surface peaks and valleys……………………………………………………………...272 Figure 7.19 – Micrographs of diamond surface of samples deposited at 240 torr with similar substrate temperatures and different methane concentrations…………………………………………………….274 Figure 7.20 – Micrographs of diamond surface of samples from Figure 7.14, which were deposited at 240 torr with different methane concentrations…………………………………………………....276 Figure 7.21 – Micrographs of diamond surface of samples from Figure 7.15, which were deposited with methane concentration of 5% at different pressures………………………………………………..278 Figure 7.22 – SCD linear growth rate and Raman FWHM versus substrate temperature for Reactor B. Experimental conditions: pressure = 240 torr, CH4/H2 = 6%, Zs = -3.34 mm – -5.73 mm………..…281 Figure 7.23 – SCD linear growth rate and Raman FWHM versus substrate xxiv temperature for Reactor B. Experimental condition: pressure = 240 torr, CH4/H2 = 5%, Zs = -5.73 mm – -7.54 mm…………..282 Figure 7.24 – Transmission measurement results. Experimental conditions: 160 torr, CH4/H2 = 5% (Reactor A); 240 torr, CH 4/H2 = 6% (Reactor B); 240 torr, CH4/H2 = 5% (Reactor C)………….…..286 Figure 7.25 – Examples of micrographs with light and from birefringence imaging for Reactors A and B…………………………………...288 Figure 7.26 – Process steps of large area SCD substrates synthesis by the side-surface growth method. The small arrows indicate the diamond growth directions…………………………………..…..296 Figure 7.27 – Example of micrographs of a sample after side-surface synthesis. Experimental conditions: pressure = 240 torr, CH4/H2 = 7%, Zs = -3.21 mm. (a) 25x magnification, (b) 100x magnification….…298 Figure 7.28 – Example of micrographs of a sample after 24-hour deposition. Experimental condition: pressure = 240 torr, CH4/H2 = 6%, Zs = -3.34 mm. (a) 25x magnification, (b) 100x magnification….....301 Figure 7.29 – Micrographs of an example sample after 3 resumption synthesis experiments. Experimental conditions: pressure = 240 torr, CH4/H2 = 7%, (a) Zs = -4.49 mm, (b) Zs = -3.34 mm, (c) Zs = -3.21 mm…………………………………………..……….……………..303 Figure 8.1 – Linear growth rate versus area power density for different CVD diamond deposition reactors [76]. 1. HFCVD; 2. conventional DC CVD; 3. enclosed DC Arcjet CVD; 4. Atmosphere DC Arcjet CVD; 7. RF thermal plasma CVD; 8. magneto-microwave CVD; 9. Tubular microwave CVD; 10. Microwave plasma jet CVD; 11. ASTeX bell jar microwave CVD; 12. MSU bell jar microwave CVD; 13. MSU bell jar microwave CVD (high pressure). The gray crosshatched area indicates the location where the SCD synthesis results from this dissertation research are located on the figure…………………………………………………………....308 xxv Figure A.1 – Schematic drawing of a pocket substrate holder for single crystal diamond synthesis experiments. The dimensions summarized in Table A.1 are indicated in this figure…………………………..…315 xxvi CHAPTER 1 INTRODUCTION 1.1 Research motivation The major motivation of this dissertation research was to develop microwave plasma assisted chemical vapor deposition (MPACVD) technologies and processes that efficiently enable high growth rate, high quality single crystal diamond (SCD) synthesis in the high pressure, 180 to 280 torr, regime. An existing MPACVD reactor, identified here as Reactor B, was developed to grow diamond at high pressures and high power densities [1]. Kadek W. Hemawan designed and built Reactor B as part of his thesis research [1]. He then evaluated Reactor B’s performance by investigating polycrystalline crystal diamond synthesis over the pressure regime of 180-240 torr. This dissertation research will continue the experimental evaluation of Reactor B at high pressures by investigating SCD synthesis in the pressure range of 180-280 torr. During the past several decades, CVD diamond deposition has been considered as a very promising method for commercially synthesizing diamond [102, 65]. Compared with other CVD diamond synthesis technologies, the MPACVD diamond synthesis process provides stable conditions, 1 reproducible sample quality with reasonable cost. In 2002, C.S. Yan et al. [2] originally reported that MPACVD SCD can be produced with high quality and with high growth rates (50-150 μm/h) at high pressures of 100 to 200 torr. This revelation drew much attention to SCD synthesis at high pressures by MPACVD [4, 21, 29, 30, 32, 35, 43, 45, 103, 104]. Since then, they in 2009 further demonstrated MPACVD SCD synthesis with high growth rates and high quality at pressures up to 350 torr [3]. They reported a high growth rate of 165 μm/h at 300 torr (with several thousands ppm N2 addition). Their achievement demonstrates the promise of high growth rate, high quality SCD synthesis with very important commercial potential. However, in their publications, they have not provided much experimental process details. In particular they have not published the exact range of experimental conditions required to achieve excellent SCD synthesis. On the other hand, theoretical and numerical analysis suggests that the growth rate and quality of diamond should increase as pressure and microwave discharge absorbed power density are increased [4, 10]. F. Silva et al. [4] have developed plasma models to compute the species concentrations in the gas phase as a function of deposition pressure, and showed that there is an associated large increase in atomic hydrogen density [H] and CH 3-radical density [CH3] at high pressure. Thus at high pressure the radical density conditions are expected to benefit both diamond growth rate and quality [4, 66]. 2 An objective of this dissertation research was to investigate and provide experimental details of MPACVD SCD synthesis at high pressures and high power densities. The SCD synthesis experiments in the dissertation only utilized Reactor B. One goal of this dissertation research was to experimentally determine the experimental conditions that are required to safely, efficiently and stably operate Reactor B within the high pressure regime of 180-280 torr, i.e. experimentally identify the safe and efficient, high pressure SCD experimental operating regime for Reactor B. This was achieved as described in Chapter 5 of this dissertation by experimentally determining the operating field map for Reactor B within the high pressure regime. Another goal of this dissertation research was to explore the influence of several input experimental variables on MPACVD SCD synthesis. The MSU seven-inch CVD Reactor B was experimentally evaluated at high pressure (180-280 torr) and high power densities by synthesizing SCD over a wide range of experimental conditions. The range of experimental conditions included: (1) pressures of 180-280 torr, (2) absorbed microwave power 3 densities of 200-700 W/cm , (3) methane concentrations of 4-7%, (4) substrate temperatures of 950- 1300 ° (5) extra nitrogen concentration of C, 0-200 ppm. High quality SCD synthesis was demonstrated as Reactor B operated within the desirable, high pressure, and high discharge power density variable space. A number of characterization techniques were performed on the synthesized SCD samples to determine the diamond quality. As predicted 3 by the theory [4, 66], the high discharge power densities produced the high species concentrations on the substrate surface that improved the diamond quality and increased the deposition rate. Thus a desirable experimental multivariable process parameter space within the high pressure regime, i.e. a safe, efficient and high quality SCD process window, was identified for Reactor B. 1.2 Research objectives and approach The main objectives of this research were to evaluate the existing MPACVD Reactor B at high pressures of 180-280 torr by experimentally investigating SCD synthesis, and experimentally defining the safe, efficient and stable process window for high growth rate high quality SCD production. The specific tasks of this dissertation research are outlined below: Task 1: Extend the research of Kadek W. Hemawan [1] on Reactor B and focus on MPACVD SCD synthesis in the high pressure regime (180-280 torr). Task 2: Develop an experimental methodology that defines the safe and efficient experimental operating regime for Reactor B. Task 3: Experimentally determine the operating field map for Reactor B in the high pressure regime. Task 4: Explore the influence of several important input experimental 4 variables on SCD synthesis. Task 5: Identify a high quality high growth rate SCD synthesis window. Task 6: Demonstrate high quality SCD plate synthesis. Task 7: Begin the experimental exploration of large area SCD substrate synthesis process. 1.3 Dissertation outline This dissertation consists of 8 chapters. Chapter 2 presents the current theoretical understanding of MPACVD diamond synthesis and then reviews recent research results concerning SCD synthesis by MPACVD. In Chapter 3, the experimental systems that were used in this dissertation research are described. Chapter 3 includes descriptions of the MPACVD Reactor B and associated sub-systems, such as the gas flow control and the vacuum systems. The work that was done by previous students using this Reactor B is also summarized. Chapter 4 begins with a description of the SCD substrate seeds and the pre-treatments and post-treatments of the substrates. It also presents the details of the experimental operational procedures associated with experimental systems, and the methods of diamond characterization. The multi-dimensional experimental variable space is summarized in Chapter 4 as well. The experimental methodology that defines the safe and efficient experimental reactor operating regime is presented in Chapter 5. This includes 5 a discussion about the photographic techniques and power density calculations. The discharge behavior of Reactor B at high pressures is explored and presented. The operating field map for Reactor B is experimentally determined, and then the safe and efficient experimental operating region is defined. Chapter 6 describes the details of some methods of diamond synthesis process control that were developed and used in this thesis to synthesize SCD. Specific details of the process control methods, such as substrate temperature control versus time, process control via substrate position variation, and reactor matching via Ls adjustment, etc. are described. Chapter 7 presents the experimental evaluation results of the SCD synthesis using Reactor B. An experimental study of a hydrogen etching pre-treatment process for SCD substrates is also presented in this chapter. The experimental results of SCD synthesis is discussed within the vast experimental variable space. In particular, the relationship of input variables, such as substrate temperature, pressure, and methane concentration, and output variables, such as growth rates, and diamond quality is investigated and analyzed. A high quality, high growth rate SCD synthesis window versus is experimentally identified. Furthermore, the large area SCD plate production is briefly experimentally explored. Chapter 8 concludes the dissertation with a summary of SCD synthesis using Reactor B. research are also presented. 6 Recommendations for future CHAPTER 2 MICROWAVE PLASMA ASSISTED CVD DIAMOND SYNTHESIS BACKGROUND 2.1 Introduction This chapter presents a general background and a literature review of microwave plasma assisted chemical vapor deposition (MPACVD) diamond synthesis. Section 2.2 briefly describes a basic understanding of MPACVD diamond synthesis, including a description of the properties of diamond and its potential use in various applications, and the general growth process and surface kinetics chemistry of CVD diamond. It also identifies the main growth species in the diamond deposition process, especially at high pressures (> 120 torr). The review of literature of recent achievements in MPACVD single crystal diamond (SCD) synthesis is presented in section 2.3. 7 2.2 Introduction of single crystal diamond synthesis by MPACVD 2.2.1 Properties of diamond As compared to other materials, diamond has unique and superior properties. Diamond’s highly attractive properties make it a very promising material for various technical applications. Among other properties, diamond is the hardest known solid material, has a very high bulk modulus, which makes it widely used as abrasive coatings for cutting or drilling tools. It has highest thermal conductivity at room temperature, broad optical transparency from the deep ultraviolet to the far infrared, and highest sound propagation velocity, which enables diamond to be used for many applications, such as heat sinks for electronic devices, optical windows and surface acoustic wave devices. Diamond is a very good electrical insulator and can be doped to become a semiconductor, which means that diamond has the potential to be used as an electronic material. Diamond has a wide bandgap of 5.45 eV, and a high electric breakdown field of 10000 kV/cm. Semiconductor devices made of diamond would have higher breakdown voltages, and have lower specific on-resistance. They may operate in higher radiation environments, at higher electrical power levels, and with higher operating temperatures. Diamond also can be used as an inert conducting electrode for a variety of electro chemical 8 applications and in harsh environments, since it is very resistant to chemical corrosion and is inert to most chemical reagents. Listed below are the mechanical, thermal, optical and electrical properties of diamond [5-8]. Table 2.1 – Diamond properties. Hardness 10,000 kg/mm Strength, Tensile >1.2 GPa Sound velocity 17,500 m/s Young’s modulus 1140 GPa Atom density 1.77x10 Thermal conductivity 22 W/cm-K Refractive Index 2.41 Dielectric constant 5.68 Optical transparency UV to far IR Saturated electron velocity 2.7x10 cm/s Electron mobility 2,200 cm /Vs Hole mobility 1,600 cm /Vs Band gap 5.45 eV Electric breakdown field 10000 kV/cm Resistivity 10 2 23 cm -3 7 2 2 9 13 – 10 16 Ωcm 2.2.2 CVD diamond synthesis process The CVD diamond growth is a complicated process, and the full description of the process is rather complex. A general simplified MPACVD diamond growth process is shown in Figure 2.1 and is summarized as follows:  There usually is a substrate holder on which one to several diamond seeds (for single crystal diamond) or seeded silicon wafer (for polycrystalline diamond) as substrates for diamond deposition are located.  A microwave plasma activation region is located closely above the substrate and substrate holder.  Initially the feed gases (H2 and CH4 for single crystal and polycrystalline diamond) first mix in the plasma activation region before diffusing through the plasma towards the substrate surface.  On the way to the substrate, input gases pass through an electric discharge activation region that provides energy to the gaseous species. This activation results in the breaking down of the gases molecules into reactive radicals and atoms, the creating of ions and electrons, and the heating of the gas molecules to a high temperature of the order of a couple thousand K.  As these reactive radicals hit the substrate surface, they continue to mix and undergo a very complex set of chemical reactions. At this stage, the species absorb and react with the surface, desorb back into the gas 10 phase, or diffuse around close to the surface until an appropriate reaction site is found.  When the appropriate surface reactions and all gas phase chemistry conditions are met, diamond is deposited on the substrate. Figure 2.1 – General MPACVD diamond growth process. For interpretation of the references to color in this and all other figures, the reader is referred to the electronic version of this dissertation. 2.2.3 Substrate surface chemistry in diamond growth process Diamond growth by using a hydrogen-methane mixture requires both atomic hydrogen and carbon-containing radicals. The microwave discharge serves to dissociate some of the molecular hydrogen into atomic hydrogen and to dissociate the methane molecules into CH3 and possible other diamond 11 growth radicals such as CH2 and CH [9]. The major growth species in diamond deposition process usually consist of carbon containing radical such as C, H, CH, C2, C2H2, CH3, etc. Although there is currently no model that describes all aspects of CVD diamond growth, some simplified approaches provide an understanding of the gas-surface chemistry occurring during CVD diamond deposition. C.C. Battaile et al. [11] simulated the diamond growth of {110}- and {111}- oriented diamond films under typical CVD conditions on the atomic scale. The diamond growth typically begins from a surface saturated with hydrogen atoms. The substrate interacts with a gas mixture containing H, H2 and CH3 according to the set of chemical reactions specified in Table 2.2 [11]. Cd represents a surface diamond atom, and an asterisk (*) represents a surface biradical. Table 2.2 – A set of reactions between substrate and gas mixture [11]. 1 CdH + H ↔ Cd + H2 3 Cd + CH3 ↔ CdCH3 5 CdCxHy + H ↔ CdCxHy-1 + H2 7 CdCHy + CH3 ↔ CdC2Hy-3 9 Cd + * + CdCxHy → Cd + CdCx-1Hy + Cd Figure 2.2 shows examples of the reaction sequences leading to growth from CH3 on both {110} and {111} faces. Growth on a flat {110} surface (Fig. 12 2.2(a)) is initiated by  H abstraction from and CH3 chemisorption onto the diamond surface,  H abstraction from and CH3 addition to the chemisorbed CH3 molecule,  H abstraction from the second CH3 molecule and from an adjacent site on the diamond surface,  finally bonding between the C2H4 radical and the film. Growth on the {111} surface in Fig. 2.2 (b) occurs by  two neighboring H abstraction and CH3 chemisorption,  H abstraction from both chemisorbed CH3 molecule,  CH3 chemisorphtion at one of the newly created radical sites,  H abstraction from the last CH3,  finally bonding between the C2H4 and CH2 radicals. Figure 2.2 – Examples of reaction sequences leading to growth on the (a) {110} and (b) {111} faces of diamond. The text below the images indicates the reactions according to the labeling in Table 2.2. Dark gray circles are diamond, light gray circles are C atoms in chemisorbed hydrocarbons, and white circles are H atoms [11]. 13 In a word, starting with a surface on which all unsatisfied carbon bonds are H-passivated, an atomic H abstracts a surface H to form H2 leaving behind a reactive surface site. The open site can react with another nearby H atom, which results in the stable form as its previous situation, or a CH3 radical from the gas phase which can be added to a carbon in the lattice. This process of H abstraction and CH3 addition may then occur on an adjacent site, or on one of the newly chemisorbed sites, either of which will create a radical to reach the other nearby open site, and finally complete the diamond lattice. 2.2.4 Main radicals in diamond growth process The model proposed by Goodwin et al. [10] describes the growth of CVD diamond in a very simple way. Among all the possible diamond growth radicals, this model assumes that diamond growth involves only two species, H and CH3, and the five reaction rates as shown in Table 2.3 [4]. The diamond growth proceeds by incorporation of growth units consisting of methyl radicals. Similarly, this incorporation is made possible by the creation of open sites available for growth denoted as Cd*. From this simplified kinetic scheme, it is possible to derive an expression for the growth rate G which depends on the surface concentrations of H and CH3, i.e. densities [H] and [CH3] [4]: G = k3 * (ns / nd) * (k1 / (k1 + k2)) * [CH3][H] / ((k4 / k5) + [H]), (1) where k1, k2, k3, k4 and k5 are the reaction rate constants as indicated in Table 2.3, ns is the surface site density (2.61 x 10 14 -9 2 mol/cm on (100) surfaces) and 3 nd is the molar density of diamond (0.2939 mol/cm ) [9]. 3 Table 2.3 – A reduced methyl diamond growth mechanism. Units for k: cm , mo, s [10]. Reactions Reaction rates (k) 1. CdH + H → Cd* +H2 k1 = 2.9 x 10 2. Cd* + H → CdH k2 = 1.7 x 10 3. Cd* + CH3 → CdCH3 k3 = 3.3 x 10 4. CdCH3 → Cd* + CH3 k4 = 1.0 x 10 5. CdCH3 + H → CdCH2 + H2 k5 = 2.0 x 10 12 13 12 4 12 * Arbitrarily set. Using the k values in table 2.3 leads to 11 G = (1.8 x 10 ) * [CH3][H] / (5 x 10-9 + [H]). (2) This equation provides a simple relation which empirically predicts measured growth rates with reasonable accuracy [10]. The challenge in CVD diamond deposition with higher rates is to do so while maintaining high film quality, i.e. low defect density. To do this requires an understanding of how defect generation depends on the local chemical environments. According to the results obtained from Goodwin’s model [10], at constant substrate temperature, the relative defect density may be estimated by the following formula: Xdef 2 G/[H] . (3) Thus, the diamond film growth rate and quality are both determined by 15 densities of H and CH3. Using the simplified model, the radicals at the substrate surface are the most important radicals in the CVD diamond growth process. F. Silva et al. [4] have developed numerical plasma models to compute the [H] and [CH3] in the gas phase as a function of the deposition parameters, e.g. pressure, microwave power and methane concentration. They noticed that in their reactor the electron temperature drops from 15000 to 10000 K, and the gas temperature varies from 2200 to 3600 K as the pressure increases from 37.5 torr to 225 torr. This is due to the increase in electron-heavy species collision frequency which results in an enhanced energy transfer from the electron gas to the heavy molecule species. They pointed out that, at low pressure (< 100 torr), H is mainly produced by the direct electron-impact dissociation reaction - - e + H2 → e + 2H. However at higher pressure, H is mainly produced by the thermal dissociation of H2 molecules, i.e. H2 + H → 3H. This result clearly shows that as pressure increases the gas temperature increases more than 1400 K, and then there is an associated large increase in atomic hydrogen density [H]. The modeling results also shows that at low pressure (< 100 torr), CH3 radical production in the plasma bulk is governed by H-atom concentration 16 through the reaction CH3 + H + M ↔ CH4 + M where M is a third body. At high pressure, CH3 production is still due to CH4 dissociation through collisions with H atoms through the following reaction CH4 + H ↔ CH3 + H2. This reaction mainly takes place very close to the substrate surface. In fact, the production rate of CH3 also depends on the gas temperature. An optimal CH3 production is obtained for gas temperatures of 1200-2200 K. They calculated for their MPACVD reactor the operating points in terms of H-atom and CH3-radical concentrations for different pressures and input power discharge conditions and then plotted them on the process map from Goodwin [10] in [CH3]-[H] space showing the operating ranges of the main CVD diamond growth processes. This is shown in Fig. 2.3. This diagram shows the lines of constant growth rate and quality based on the equations (2) and (3) described above for their reactors. For given methane concentration, the increase of the pressure from 37.5 to 225 torr leads to three orders of magnitude increase of the H-atom concentration and one order of magnitude increase of CH3 radical concentration. This results in an increase of growth rate to several tens of μm/h. As one would expect, at a constant pressure, the relative defect density with a 8% methane concentration is higher than that with a 1% methane concentration. However, for a constant methane concentration, the diamond quality can be improved by increasing the 17 operation pressure. Thus it shows that increasing methane concentration at high pressure leads to a significant increase of the growth rate without compromising the film quality. This implies that at higher pressure the growth rate increases and high quality diamond can be synthesized with higher methane concentrations than at lower pressure. The high diamond quality, high growth rate synthesis window increases with high pressure operation. Figure 2.3 – Process map in [CH3]-[H] space showing the operating ranges of the main CVD diamond growth processes [10]. The evolution of the operation point of the MPACVD reactor from LIMHP as a function of working pressure and methane concentration has been added [4]. 18 In summary, the process map shows all the advantages of synthesizing diamond at high pressures. This allows increasing the diamond growth rate and the film quality simultaneously, the two aspects which attract most attention in CVD diamond synthesis. This is the reason that the research activities in this dissertation were focused on MPACVD single crystal diamond synthesis in the high pressure regime (> 180 torr). 19 2.3 Literature review of MPACVD single crystal diamond synthesis This literature review of the microwave plasma assisted CVD (MPACVD) diamond synthesis is focused on the synthesis of single crystal diamond (SCD). Recent world wide SCD synthesis activities are summarized below by research groups. Most of the work summarized has occurred after year 2007. Earlier SCD research work results have already been described in previous research dissertations at Michigan State University. For example see ref [1]. 2.3.1 Carnegie Institution of Washington, U.S.A. The Carnegie Institution group has focused on the growth of large volume SCD for use in many applications, such as anvils in high-pressure research. Over the past decade, they have achieved important advances in large SCD synthesis at high growth rates by MPACVD [2, 3, 12-19]. They reported high growth rates from 50 to 150 µm/h [2], which at the time was up to two orders of magnitude higher than standard CVD processes for making SCD. The types of gas chemistry and growth conditions, including stage design, microwave power, pressure, and substrate surface temperatures, were varied to optimize diamond quality and growth rates. They produced SCD crystals with a weight of over ten carats and thicknesses over 1 cm at growth rates of 50-100 µm/h and at pressures up to 300 torr [3, 12-14]. Colorless and near 20 colorless single crystals up to two carats were produced by further optimizing the process [3, 12, 13]. The diamond was characterized by a variety of spectroscopic and diffraction techniques. Spectroscopy measurements verified that the colorless crystal quality was at least comparable to high grade, type IIa diamond. By developing a new post-growth treatment processes, they have further enhanced the mechanical and optical properties of the synthesized SCD [3, 15-17]. Their recent achievements in high growth rate SCD synthesis are summarized below. 2.3.1.1 Large single crystal diamond fabrication with high growth-rate They reported that the small addition of nitrogen to the gas synthesis enhanced growth, leading to synthesized diamond with a yellowish or light brown color due to the broad UV-visible absorption [2]. They claimed that the SIMS measurements showed that the typical nitrogen content was below 10 ppm, and the diamond quality was still comparable to that of type IIa diamond. On the other hand, the oxygen added to the CVD diamond-growth chemistry can also help facilitate diamond formation [3]. New oxygen related species 2 formed in the plasma etch away impurities (hydrogen, and sp carbon) and defects on growth surface more efficiently than atomic hydrogen, and therefore improve the diamond quality. 21 The improved growth techniques for producing colorless SCD have been summarized generally as follows [3].  Reactor type: ASTeX/Seki reactor.  Substrate seeds: HPHT synthetic type Ib and synthesized SCD with {100} surfaces with minimum surface defects were used as a substrate for diamond growth.  Substrate pre-treatment: cleaned ultrasonically with acetone.  Gas chemistry: 5-20% CH4/H2, 0.2-10% O2/CH4. A hydrogen generator with palladium purifier was used to produce clean hydrogen with 7N purity. High purity methane (99.9995%) was also used.  Pressure: 150-300 torr.  Input power: 3-5 kW.  Discharge power density: not provided.  Substrate temperature: 1000 ° to 1500 ° C C. The deposition temperature was measured by a two-color infrared ratio thermometer [2].  Growth rate: 40-100 µm/h. Near colorless and colorless diamond crystals were synthesized and polished, after cutting by a computer controlled Q-switched YAG laser system. Three representative samples are shown in Figure 2.4. The general growth conditions can be found in Table 2.4 [12]. Specific detail information, such as growth pressure, CH4 concentration, substrate temperature, and microwave 22 power were not provided. Figure 2.4 – Selected near colorless and colorless SCD specimens, clockwise from the top: (1) SCD-1: light brown, brilliant cut and polished single crystal containing nitrogen (~0.5 carat); (2) SCD-2: near colorless, 0.2 carat brilliant cut and polished single crystal produced from a ~1 carat block; (3) SCD-3: colorless 1.4 carat bullet shape single crystal produced from a ~2.2 carat block [3]. 23 Table 2.4 – Growth parameters for SCD specimens shown in Figure 2.4 [12]. Color Pressure Growth Weight, Weight, (torr) rate before after (μm/h) N2/CH4 lasercutting lasercutting (carat) (carat) SCD-1 Brown 2% 200-220 100-120 1.2 0.54 SCD-2 Near 0.02% 220-250 75-95 0.75 0.19 250-300 50-70 2.1 1.4 colorless SCD-3 Colorless 0% These representative samples have been characterized by a variety of spectroscopic and diffraction techniques [3]. Photoluminescence (PL) spectra measured for nitrogen-doped synthesized SCD show signatures of obvious 0 − nitrogen–vacancy centers at 575 (NV ) and 637 (NV ) nm, along with a broad luminescence background (Figure 2.5). The natural type IIa diamond measured had negligible background with the prominent feature being the first-order diamond Raman peak. The second-order Raman feature between 575 and 600 nm was also observed for the type IIa diamond. Colorless synthesized SCD grown without nitrogen exhibits similar PL spectra to natural type IIa diamond. Natural type IIa and this colorless single-crystal CVD diamond are similar and difficult to distinguish. The optical properties of these materials have been investigated by UV–visible absorption spectroscopy as shown in Figure 2.6. Nitrogen-doped single-crystal CVD diamond exhibits 24 features typical of brown CVD diamond, including broad bands at 270 nm (substitutional nitrogen), 370 nm and 550 nm (nitrogen vacancy centers). There was also no significant difference in the Raman lineshape of colorless single-crystal CVD diamond and the type IIa natural diamond, though they didn’t show a comparison in the longer wavelength range. It is worth noting that the absorption coefficient of colorless synthesized SCD is slightly higher, which is likely due to nitrogen impurities originating within methane and oxygen gases used for the growth. In a word, the quality of the colorless single crystal diamond is verified comparable to type IIa diamond. Figure 2.5 – Photoluminescence (PL) spectra of natural IIa diamond, light brown, and colorless synthesized SCD (300 K) [3]. 25 Figure 2.6 – UV-visible absorption spectra of natural type IIa diamond, nitrogen doped light brown and colorless synthesized SCD [3]. 2.3.1.2 Enhanced mechanical properties Carnegie researchers have shown that the hardness of synthesized SCD can be enhanced by at least 50% (> 150 GPa) by means of high pressure high temperature (HPHT) annealing [15]. Later, they demonstrated that this method can also improve the toughness of this material [3]. More recently, they have introduced boron in the gas chemistry to enhance the fracture toughness of single crystal diamond without sacrificing its high hardness [16]. Further, 26 they found that low pressure high temperature (LPHT) annealing can enhance the intrinsic hardness of synthesized SCD [16]. The boron-containing SCD was synthesized under following conditions [16]:  Reactor type: ASTeX/Seki reactor.  Substrate seeds: HPHT synthetic type Ib diamond plates (5 mm x 5 mm x 0.3 mm) with {100} top plane.  Substrate pre-treatment: cleaned ultrasonically with acetone.  Gas chemistry: 5-20% CH4/H2, 0-0.2% N2/CH4. Inert hexagonal boron nitride powder (h-BN) was selected as the dopant and introduced into the CVD system by placing in between the molybdenum holders used to hold the diamond substrate. The amount of h-BN involved in the reaction can be determined by measuring the weight loss of the h-BN powder after the diamond growth.  Pressure: 150-220 torr.  Input power: not provided.  Discharge power density: not provided.  Substrate temperature: 1100-1300 ° C.  Growth rate: 20-100 µm/h, which is a 10-100 times improvement as compared with other boron doped single crystal diamond growth. Post-treatments were the HPHT annealing and LPHT annealing. In HPHT annealing process, the selected synthesized SCD was annealed at 27 2000 ° and 5-7 GPa for 10 min using a belt-type apparatus [3, 15]. The LPHT C annealing used the same 6 kW 2.45 GHz MPACVD reactor that was used for diamond growth. The annealing was carried out with a measured diamond surface temperature of 1600-2200 ° at gas pressures between 150 and 300 C torr [3, 16, 17]. The details about gas chemistry in the LPHT annealing process were not provided. A Vickers micro-hardness tester was used to evaluate the hardness and the fracture toughness of the annealed SCD. By measuring the length and height of the indentation, the hardness and fracture toughness of the test diamond was determined. Hardness-fracture toughness data are plotted in Figure 2.7 for natural Ia, IIa, synthetic Ib, single-crystal CVD (SC-CVD), boron/nitrogen co-doped SC-CVD (B/N SC-CVD) diamonds, HPHT and LPHT annealed SC-CVD diamonds. As shown in Figure 2.7, three zones are defined: on the left are the reported and measured values for natural type Ia and IIa diamond; the center zone includes type Ib and CVD diamond; the right zone includes boron doped CVD diamond. It is clear that boron doping of SC-CVD diamond significantly improves the fracture toughness by at least a factor of two, without compromising the hardness. The measurements further reveal that the LPHT annealed synthesized SCD exhibits ultrahard characteristics without an appreciable reduction in toughness. 28 Figure 2.7 – Vickers hardness and fracture toughness on [100] faces of various diamonds in the {100} directions [16]. In summary, Vickers indentation tests have revealed that the intrinsic fracture toughness of synthesized SCD can be dramatically enhanced by selective doping with nitrogen and boron, and the post-growth treatment redistributes the vacancies and impurities to improve the mechanical properties. The low cost, large area LPHT annealing process may be an alternative to HPHT annealing. 29 2.3.1.3 Production of high optical quality single crystal diamond The effects of HPHT annealing which was mentioned in the section above to alter the optical properties of synthesized SCD have significant implications for application of the material [18]. Traditionally, high temperature treatment (e.g. >1600 ° of natural single crystal diamond requires high C) pressure in order to prevent graphitization. As described above, though the extent to which the synthesized SCD material studied has lower tendency toward graphitization due to a different defect structure and concentration remains to be determined. High temperature annealing up to 2200 ° has C been conducted on CVD diamond below atmospheric pressure. Dramatic enhancements in optical properties of synthesized SCD were found without significant graphitization [3, 17]. As shown in Figure 2.8, the LPHT treatment described in the section above can produce dramatic changes in optical properties of the synthesized SCD. Large decreases of UV–visible absorption were documented. After annealing, the absorption coefficients were lowered by factors of 2 to 6, depending on the temperature. In terms of gemological color grades calibrated and quantified by absorption spectra, the optical properties improved on average by 3 grades (e.g. from J to G) [3, 17]. 30 Figure 2.8 – UV-visible absorption spectra of synthesized SCD (300 K). (a) before annealing, (b) after annealed at 1800 ° for 2 min. The inset shows C several examples of annealed diamond plates [17]. Recently, they improved the SCD synthesis technology to demonstrate the fabrication of colorless, multicarat CVD diamond at high growth rates without annealing [13]. The experimental methods are summarized as following:  Reactor type: ASTeX/Seki reactor.  Substrate seeds: HPHT synthetic type Ib diamond (9 mm x 9 mm).  Substrate pre-treatment: cleaned ultrasonically with acetone.  Gas chemistry: 8-22% CH4/H2 (H2: 7N purity; CH4: 5-5N purity). 31  Pressure: 100-200 torr.  Input microwave power: 3-5 kW.  Discharge power density: 50-100 W/cm .  Substrate temperature: 1100-1300 ° C.  Growth rate: 50-100 µm/h. 3 They performed the growth in multiple runs. The crystal was taken out to remove the polycrystalline diamond between runs and resumed the growth again. This careful control of growth was believed helpful to prevent cracks. However the details of the process were not provided. An example of a diamond produced by the above technique is shown in Figure 2.9. The large crystal was cut from a 13.5 carat rough diamond block grown at around 50 µm/h in the absence of impurities other than hydrogen. Optical microscopy revealed that the CVD diamond is clear and relatively free of inclusions and cracks. There is no visible layers (or growth interfaces) and striations from cross-sectional view within the thickness above 5 mm. The as-grown, 2.3-carat, brilliant cut diamond was color designated as near colorless. It also has high optical quality and clarity without visible layers. 32 Figure 2.9 – The picture on the right shows a 2.4 carat synthesized SCD compared with 0.25 carat synthesized SCD. The pictures on the left is an example of the evolution of synthesized SCD sample (A to B to C to D) starting with 13.5 carat crystal block (A) to the 2.3 carat cut gem anvil (D) [13]. Though shown in optical spectra measurements, there is residual color in the anvils, which may be due to the impurity in source gases and possible air contamination in the CVD system, the present results create the prospects for rapid growth, high optical quality synthesized SCD anvils over 1 carat to be routinely fabricated. The material is thus an excellent high pressure optical window, thereby extending current applications of CVD diamond for high pressure research. 33 2.3.2 Diamond Research Center, AIST, Japan The Diamond Research Center of Japan National Institute of Advanced Industrial Science and Technology (AIST) has been working on the homoepitaxial growth of diamond since 2004 [20-41]., They have achieved high growth rates which ranged from 30 to 150 µm/h at pressures up to 220 torr with nitrogen additions of over 0.1% [20-22]. By altering the conventional reactor from SEKI Technotron Corp, a new MPACVD reactor design was realized and high diamond growth rates were achieved over an area with a 1-inch diameter, while the input power is kept less than 5 kW [24-29]. Recently a process consisting of (1) a high rate SCD growth and (2) lift-off process using ion implantation was developed [30, 31]. Combining with the side-surface growth and the “mosaic wafers” or “tiled clones” growth steps, large area SCD wafers with 1-inch size were successfully fabricated [32-37]. Their recent achievements are summarized as follows. 2.3.2.1 High rate SCD growth with nitrogen addition The high rate SCD growth was performed using a conventional [20-22] 2.45 GHz, AX-5250 5 kW-MPCVD system produced by Seki Technotron Corp. The specific experimental conditions are summarized as bellow:  Reactor type: Seki reactor.  Substrate seeds: HPHT synthetic type Ib diamond (3 mm x 3 mm x 0.5 mm). 34  Substrate pre-treatment: Hydrogen etching for 30 minutes. The typical etching depth was estimated to be 1.4 µm.  Gas chemistry: 500 sccm H2 (6N) / 60 sccm CH4 (6N) / 0.6-1.8 sccm N2 (4N) (12% CH4/ H2, 0.12-0.36% N2/ H2).  Pressure: 17-29 kPa (130-220 torr).  Input microwave power: 1-3.7 kW.  Discharge power density: 50-150 W/cm .  Substrate temperature: 1000-1300 ° C. 3 The growth temperature was measured by an optical pyrometer (Chino IR-U) and monitored by a 2-color infrared radiation thermometer (Chino IR-C) through viewing ports of the reactor. The thermometer is used only for monitoring purpose due to the strong optical emission from the plasma during the deposition.  Growth rate: 30-150 µm/h. The growth rate was estimated from the net weight gain measured by an analytical balance. In order to obtain high growth rates by enhancing the plasma density, they specially designed two new types of substrate holders. Both holders had smaller diameters than the ASTeX’s original one [20, 21], and were identified as “open type” and “enclosed type”. The holders were different geometrically as shown in Figure 2.10. The “enclosed” type supports a substrate inside a drilled hold of the Mo rod in which the top surface of the substrate is beneath 35 the top surface of the holder. The “open” type holder supports a substrate on the Mo rod, thus the substrate surface is above the top surface of the holder. Their experiment results showed that the shape of the substrate holder is one of the important factors to achieve high rate growth. The growth rates were presented for the two holders as shown in Figure 2.10. Using the open type holder, a maximum growth rate of about 150 µm/h was obtained at the reactor pressure of 29 kPa (220 torr). This was due to the enhanced discharge power density since the cross section area of substrate holder was reduced. Figure 2.10 – Plot on the left: Effect of reactor pressure on the growth rate for open and enclosed type holders [22]. Figures on the right: schematic illustration of “open type” and “enclosed type” substrate holders [21]. Using the open type holder, temperature dependence of growth rate was investigated. Diamond films were grown for about 1 h at the growth temperature range of 1060 to 1250 ° As shown in Figure 2.11, high growth C. 36 rates were obtained at the measured temperature range. They claimed that the growth rate was independent of the substrate temperate in the range of 1100-1200 ° C. Figure 2.11 – Effect of growth temperature on the growth rate for open type holder. Optical microscope image shows CVD diamond with the thickness of about 100 µm grown on a substrate. Growth temperature corresponding to each image is indicated in the plot [22]. Using a high growth rate SCD synthesis technique, very thick SCD was synthesized by a multi-step growth process. It was necessary to interrupt the growth within a limited time and then clean the substrate holder to avoid the 37 build-up of the polycrystalline diamond films on the substrate holder. It was found that using enclosed type holder could help to obtain smooth and flat surface morphology and to reduce the formation of polycrystalline diamond rim on the external edge of the substrates. Thus in the multi-step growth process, the enclosed type holder was used to grow very thick diamonds. Figure 2.12 – Large CVD diamonds grown by multi-step high growth rate synthesis process. (a) A 10 mm thick, 4.7 ct CVD diamond, (b) a 9.6 mm thick, 3.5 ct CVD diamond, (c) an 8.7 mm thick, 4.4 ct CVD diamond [22]. A result is shown in Figure 2.12. Large diamond crystals with thicknesses of 8.7-10 mm and weight of 3.4-4.65 ct were successfully grown 2 on a 27-37 mm seed after 24-31 separate deposition steps. The growth temperature was 1130 ° and the pressure was 21 or 24 kPa (150 or 180 torr). C The flow rate of nitrogen was 0.6 sccm (0.12% N2/ H2). The average growth 38 rate was 52-68 µm/h. Even though the side surface was covered with polycrystalline diamond, the top surface of the synthesized SCD maintained smooth and flat surface morphology. By investigating the Raman spectra over a cross section that was cut from one large synthesized SCD crystal, the thick -1 diamond crystal was showed to be of high quality (FWHM < 1.85 cm )[23]. 2.3.2.2 Numerical and experimental studies of a new discharge reactor The AIST group has numerically investigated the variation of the geometrical structure of microwave plasma CVD (MWPCVD) reactor, and evaluated the reactor designs over a 1-inch diameter substrate with input powers of less than 5 kW. The numerical and experimental studies were conducted for both conventional and new reactor configurations [24-29]. Figure 2.13(a) shows the schematic cross-sectional view of the conventional reactor, which is the AX6500 produced by Seki Technotron Corp.. They proposed a new concept of the MWPCVD reactor which was expected to enhance the power-efficiency of the growth. This was partially realized by altering the conventional structure as shown in Fig. 2.13(b), where the top-wall of the chamber was replaced by a cylindrical, copper conductor. They claimed that this conductor acted as an antenna, and then the microwave discharge was formed only in the narrow gap between the bottom surface of the antenna and the substrate. Actually they changed the top cylindrical wave guide section shown in Figure 2.13(a) into the coaxial waveguide section shown in Figure 39 2.13(b). Figure 2.13 – Schematic illustration of the cross section of the reactor designs: (a) conventional configuration, (b) new configuration [28]. For the conventional reactor design configuration, the numerical simulation showed that the intense plasma discharge region becomes hemi-spherical and the discharge region is reduced as pressure is increased (Figure 2.14(b) and (c)). At lower pressure (Figure 2.14(a)), the discharge is concentrated around the holder-edge. For the new configuration, the lateral uniform coverage of the plasma is relatively broad in the case of lower pressure (Figure 2.15(a)). For the elevated pressure, the coverage is limited to the region only around the substrate, but does not become narrower than 1-inch in diameter (Fig. 2.15(b) and (c)). Even for 180 torr, electron number density is very high over the area. 40 Figure 2.14 – Contours of electron number densities for the conventional configuration. Gas pressures are set to (a) 80 torr, (b) 110 torr, and (c) 150 torr, respectively [28]. 41 Figure 2.15 – Contours of electron number densities of for the new configuration. Gas pressures are set to (a) 80 torr, (b) 110 torr, and (c) 150 torr, respectively [28]. By using multiple samples for deposition, they conducted experiments to estimate the variation of the growth-rates versus radius for the new reactor 42 design. Figure 2.16 shows a photograph of the discharge with an input power of 4.5 kW and pressure of 150 torr. As shown in this figure, three Ib-type SCD substrates (3 mm x 3 mm x 0.5 mm) were placed from the center to the edge of the holder (diameter = 26 mm) at even intervals. In this case, the source gas consisted of 890 sccm hydrogen, 107 sccm methane (10%) and 3 sccm nitrogen (0.3%). The discharge was maintained for 1 h with the substrate temperature maintained around 1100 ° C. Figure 2.16 – Photograph of the discharge region in the horizontal direction [27]. The obtained growth rates of these three samples are given by open squares in Figure 2.17, where the left vertical axis represents the growth rate and the horizontal axis represents the radial coordinate. Growth rates of over 55 µm/h were obtained over the area with a diameter of 1 inch. The growth rate of the substrate at the edge was around 70 µm/h, and was higher than 43 that of the central substrate. The solid line with the solid circles in Figure 2.17 shows the simulated profile of the power density obtained for the condition that corresponds to that of the experiment. Similar to the spatial variation of the growth rate, the power density is also large in the edge region. Figure 2.17 – Radial distribution of the growth rate (open squares; left vertical axis) that was experimentally obtained and the power density (solid line with solid circles; right vertical axis) that was numerically obtained. The horizontal axis represents the radial coordinate [27]. 44 2.3.2.3 Synthesis of large single crystal diamond plates by lift-off process Usually the fabrication of SCD plates involves a conventional cutting technique. However, using this laser cutting technique, diamond material losses increase for plates with larger cross sections. A life-off process using ion implantation was recently developed to produce large and thick SCD plates and reduce material losses [30-39]. As shown in Figure 2.18, a high energy ion beam was injected onto the top surface of the seed substrate, resulting in the formation of a graphitic layer beneath the top surface (left two images in Figure 2.18). After this, a SCD layer was grown on the top surface of the substrate by using the high growth rate diamond synthesis technique as is described in the section above (third image from left in Figure 2.18). By selectively etching away the graphitic layer [42] using a noncontacted electrochemical etch method, a grown SCD plate was obtained as a freestanding wafer (right hand image in Figure 2.18). Because the thickness of the graphitic layer and depth of this layer from the top surface are very small, several micrometers or less, the loss of material from the seed crystal was negligible. Figure 2.19 shows an example of a separated CVD diamond plate with the thickness of 0.4 mm together with the HPHT substrate. 45 Figure 2.18 – Lift-off process with ion implantation [36]. Figure 2.19 – A 10 mm x 10 mm x 0.4 mm synthesized SCD plate (right) separated from a HPHT diamond substrate (left) by the combination of high rate growth and lift-off process using ion implantation [30]. 46 Figure 2.20 – Schematic illustration of a crystal enlarging process by combination of lift-off process and side-surface growth [32]. However, there is a size limitation for the CVD diamond plates produced by the lift-off process described above. The size was limited by the size of the original seed. Then they tested a new process to fabricate larger CVD diamond plates by combining the lift-off process and side-surface growth [32]. The process steps are shown in Figure 2.20. A HPHT diamond with size of 10 mm x 10 mm was used to fabricate a single-crystal CVD diamond plate with the same cross section and a thickness of 0.4 mm by the lift-off process (steps 1 and 2, Figure 2.20). The fabricated plate was then grown to a thickness of 3.7 mm with the multi-step high growth rate diamond synthesis process with 0.12% nitrogen addition (step 3, Figure 2.20). The resultant SCD was laser cut and polished to create side {100} faces. A diamond layer of thickness of 3-4 mm was grown on three of the faces (step 4, Figure 2.20). This produced a half-inch (12.6 mm x 13.3 mm x 3.7 mm) SCD as shown in Figure 2.21. This diamond was laser cut to form two pieces of (001) seed plate. One of the 47 plates was polished and used as a substrate for the further lift-off process. A thick diamond plate was successfully grown on this seed plate and separated from the seed plate as shown in Figure 2.22. Figure 2.21 – A half-inch single-crystal diamond produced by the step 4 of process shown in Figure 2.20. Arrows correspond to the growth direction [32]. Results indicate the process in Figure 2.20 is effective for enlarging the area of synthesized SCD plates. In addition, the enlarged seed plate can be used to produce larger CVD diamond plates by utilizing the process repeatedly. 48 However, the growth must be maintained stably for a long time to obtain a 1 inch wafer. This was still not easy. Figure 2.22 – (a) Photograph of a half-inch substrate plate by cutting and polishing the diamond in Figure 2.21. (b) Photograph of a half-inch SCD plate produced with N2 addition from the substrate plate by lift-off process using ion implantation. (c) Polarized light microscope image of the half-inch single-crystal CVD diamond plate [32]. Recently a so-called “mosaic wafers” method was developed to fabricate 1-inch size diamond wafer [36, 37]. Usually the mosaic wafer seems to have had pin-holes at each corner of the connected substrate and quite obvious boundaries existing between the constituent diamond plates. They introduced an efficient way to fabricate a mosaic wafer with smooth and almost invisible boundaries. The key point of the technique is the use of identical single seed substrates which are “cloned” from the same “mother” substrate using lift-off process. The final diamond wafer made of those clones is referred as “tiled clones” [37]. 49 The procedure to produce clones and tiled clones is shown in Figure 2.23. Using the lift-off process with ion implantation described above, “clones”, i.e. thin SCD plates, were fabricated. Each “clone” SCD plate had similar crystallographic characteristics over the large cross section area as the seed substrate. For example, in steps (1-4) of Figure 2.23, four clone substrate plates are fabricated from a seed substrate with size of about 10 mm x 10 mm. These clones were aligned manually with each other to within less than 500 µm, to make a tiled clone (step 5 in Figure 2.23). Then diamond layers were grown on both sides of the tiled clone wafer. A free-standing clone of the tiled clone can be produced by using life-off process again (step 6 in Figure 2.23). Figure 2.23 – Procedure to produce clones and tiled clones [37]. An example of a typical free-standing SCD plate made of 2 x 2 “clones” is shown in Figure 2.24. The arrows in this figure indicate the locations of the boundaries between the connected clones, which are almost invisible. Four identical clones gave a free-standing wafer around 20 mm x 20 mm, and the shape of the tiled clones was almost square. 50 Figure 2.24 – Photograph of a free-standing SCD plate with size of around 20 mm x 20 mm [37]. The results show that the use of cloned substrates is very efficient in enlarging diamond plate area. However, increasing the number of the constituent diamond plates can cause degradation in quality. Thus in orders to further fabricate high quality large area SCD plates, enlargement of monolithic SCD substrate still appears to be very important. 51 2.3.3 LIMHP/CNRS, Paris, France The diamond group of Laboratoire des Sciences des Procédés et des Matériaux, France has explored the SCD synthesis both scientifically and technologically, in terms of size, purity and crystalline quality of diamond [4, 43-57]. They have investigated the effect of growth parameters specifically for the homoepitaxial deposition of high-quality thick diamond films [43-48]. Based on microwave engineering and plasma modeling, they have studied the influence of the deposition process parameters on the production rate of the growth species and CVD diamond growth kinetics, and different designs of plasma reactors [4, 49-51]. For large surface area homoepitaxial CVD diamond growth, the influence of the presence of multiple growth sectors on the substrate surface and geometric modeling of CVD diamond crystal growth was investigated [52, 53]. They have also analyzed the impurities and defects in diamond in order to achieve device-grade single crystals [54-57]. Here are their recent advances on single crystal diamond synthesis. 2.3.3.1 Plasma modeling of CVD reactors As described in previous section 2.2, they have developed plasma models to compute the [H] and [CH3] in the gas phase as a function of the deposition parameters, e.g. pressure, microwave power and methane concentration [4, 49-51]. Their work shows that the use of high microwave power density (MWPD) plasmas is necessary to promote atomic hydrogen 52 concentrations that are high enough to ensure the deposition of high purity diamond films at large growth rates. They plotted the operating point in terms of H-atom and CH3-radical concentrations for different pressures and input power discharge conditions as shown in Fig. 2.3, based on the process map in [CH3]-[H] space showing the operating ranges of the main CVD diamond growth processes from Goodwin [10]. The process map clearly illustrates the advantage of working at high pressure. This allows increasing simultaneously the growth rate and the film quality, the two aspects which attract most attention in CVD diamond synthesis. It also shows that increasing methane concentration at high pressure leads to a significant increase of the growth rate without compromising the film quality. Figure 2.25 – MPACVD reactors and their electric field distribution. Light red color indicates higher electric field. (a) Home-made metallic-chamber reactor; (b) bell-jar reactor co-developed between LIMHP and Plassys company, showed opened on the picture [57]. 53 Figure 2.25 shows two 2.45 GHz MPACVD reactors at LIMHP. Electromagnetic modeling of the reactors has been carried out and is presented in this figure. 2.3.3.2 High quality synthesized SCD growth In this study, homoepitaxial thick diamond films were grown by microwave plasma CVD at high microwave power densities for temperatures ranging from 800 ° to 950 ° and with nitrogen addition [43-48, 57, 97]. It was C C shown that the growth rate can be increased by factors of up to 2.5 by adding small concentrations (2 to 10 ppm) of nitrogen to the gas phase. The experimental conditions are summarized as below:  Reactor type: LIMHP reactor.  Substrate seeds: commercially available synthetic (100) HPHT Ib diamonds with size of approximately 3.5 mm x 3.5 mm.  Substrate pre-treatment: Wet-chemical cleaning and then a 2-hour H2/O2 plasma etching of the HPHT substrates.  Gas chemistry: purified hydrogen (9N), 2-7% highly pure methane (6N) and nitrogen. Nitrogen concentration was varied between 0 and 200 ppm.  Pressure: 100-200 torr.  Input microwave power: 2-4 kW.  Discharge power density: 60-130 W/cm . 3 54  Substrate temperature: 800-1000 ° C.  Growth rate: 5-55 µm/h. As shown in Figure 2.26, the addition of nitrogen as low as a few ppm in the gas phase resulted in a strong increase of the growth rate. At pressure of 220 mbar (165 torr), 10 ppm of N2 improves growth rates by more than a factor of 2. This enables the growth of high quality diamond plates. It was reported that a maximum deposition rate of 55 µm/h was achieved by introducing 200 ppm of nitrogen at 875 ° [46]. C Figure 2.26 (b) shows growth rates as a function of power densities, 3 illustrating that power densities above 70 W/cm are a requisite for growing thick diamond crystals [57]. For the LIMHP reactor, coupling high discharge power densities generates huge heat fluxes and overheats the reactor chamber walls. Etching of the chamber walls or windows could also lead to silicon contamination. Thus the upper limit of discharge power densities was mostly technological. It was observed that there is a coupled effect of nitrogen addition and surface temperature on the growth mechanisms [46]. In the photoluminescence spectra of those nitrogen doped samples, as illustrated in 0 Fig. 2.27, the emission from the Nitrogen-Vacancy centres (NV at 575 nm and − NV at 638 nm) was found to be higher for the samples grown at 800 ° and C 875 ° C than for the sample grown at 975 ° C. It indicates that N surface desorption is increased at higher surface temperatures and nitrogen 55 incorporation in the CVD films decreases when the growth temperature is increased. Figure 2.26 – (a) Diamond growth rate as a function of nitrogen addition in the 3 gas phase (CH4/H2 = 4%, MWPD = 95 W/cm ). Optical and PL images 56 obtained under UV light for an undoped freestanding CVD diamond crystal and a 2 ppm N2 doped. (b) Evolution of the growth rate as a function of microwave power density at a constant surface temperature (850 ° and for two different C) methane concentrations [57]. Figure 2.27 – Photoluminescence spectra obtained at 77 K using a 514 nm green laser for excitation. The corresponding diamond layers were grown with 100 ppm N2 at (a) 950 ° (b) 875 ° and (c) 800 ° For clarity, curves have C, C C. been vertically shifted [46]. The high-quality freestanding CVD diamond plates are shown in Figure 2.28. From a macroscopic point of view all the crystals have very good optical 57 quality. They are all very clear and relatively free of inclusions. The black corners of the samples in Figure 2.28 indicate that the (111) triangular corners were more defective by nitrogen than the (100)-faces. Thus the (100) top surfaces are virtually free of defects regardless of the amount of incorporated nitrogen. Figure 2.28 – Optical images of the free-standing synthesized SCD samples obtained after laser cutting and polishing, and grown with (1) no intentional N 2 addition, (2) 2 ppm of N2, (3) 4 ppm of N2, (4) and (5) 6 ppm of N2, (6) 10 ppm of N2 in the gas phase [45]. 2.3.3.3 Multiple growth sectors on the substrate surface For homoepitaxial CVD diamond growth, the substrate is an essential element which strongly influences the electronic properties of the deposited 58 films, because of the presence of dislocations on the surface and in the bulk, which can propagate during the CVD growth. By using an etching pre-treatment by H2/O2 plasma on the substrates, one can eliminate most of the surface dislocations, which have been originally induced by substrate polishing, and therefore strongly improve the quality of the grown films. This pre-treatment technique also reveals the bulk dislocations in the HPHT seed that reach the surface through the formation of etch pits. The etch pits density so obtained can then be used as a criterion to select the least defective substrates. The substrate pre-treatment has a dramatic effect on overall morphology of the growing crystal, in the case of thick films, i.e. films with several hundred microns deposited thicknesses. Without pre-treatment, the crystal morphology showed what appeared to be excess etched {111} planes [52]. HPHT lb substrates used for epitaxial growth experiments are obtained from larger crystals containing several growth sectors as shown in Fig. 2.29. Because of this, the two substrate sides are not equivalent since one of them is composed mainly of a single <100> growth sector, while the other is made up of several different sectors. These different growth sectors exhibit large disparities in their impurity concentrations. For instance, <111> sectors are well known to incorporate much more nitrogen than other sectors. The growth sectors can be highlighted in several ways. Luminescence techniques, or an etching step, can reveal the different sectors. More specific 59 fluorescence techniques, such as DiamondView, also can show these sectors. An even simpler and more effective technique to show these sectors is scanning electron microscopy using high probe current. As illustrated in Figure 2.30, the multi-sectors of the substrate before growth can be revealed by several methods. Figure 2.29 – Depiction of the {100}, {111}, {110}, and {113} growth sectors (bottom row) in HPHT diamond substrates, which are cut from an initial HPHT crystal with multiple sectors (top row) [52]. Figure 2.30 – Highlighting the growth sectors of HPHT substrates by: a) H2/O2 plasma etching pre-treatment; b) DiamondView imaging, and c) scanning electron microscopy with a large probe current (>1 nA). The d) panel lables the 60 identified growth sectors (same colour scheme as in Figure 2.29) [52]. By examining the characteristics of the film deposited on the single-sectored substrate, it was noted, by simple visual inspection, that all four film corners are well defined and remained intact during cutting and polishing [52]. These experiments demonstrate that, when using HPHT substrates, only faces consisting of a single <100> growth sector are suitable for homoepitaxial CVD diamond growth, if one wishes to avoid as much as possible the presence of dislocations during the growth. 61 2.3.4 Institute of Applied Physics RAS, Nizhny Novgorod, Russia A research group from Institute of Applied Physics RAS, Russia has worked on the microwave plasma assisted CVD diamond growth for both polycrystalline diamond and single crystal diamond [58-64]. For single crystal diamond synthesis, they studied the CVD deposition process using high 3 microwave power densities (MWPD) of 80 up to 200 W/cm . The discharge power density was increased by increasing the gas pressure and by altering substrate holder configuration [58-60]. Furthermore, at equal power density, is was found that the growth rate was higher if diamond was grown in pulsed wave (PW) regime than that in continuous wave (CW) regime [61, 62]. 2.3.4.1 Single crystal diamond growth The design of the microwave plasma reactor used in their work is the same as Reactor A which was developed at the Michigan State University. The experimental setup is shown in Figure 2.31, and the experimental conditions are listed bellow:  Reactor type: MSU Reactor A.  Substrate seeds: (100) HPHT synthetic type Ib single crystal diamond seeds (2.5 mm x 2.5 mm x 0.3 mm).  Substrate pre-treatment: not provided.  Gas chemistry: 200 sccm H2 / 2-24 sccm CH4 (1-12% CH4/ H2). Gas purities were not provided. 62  Pressure: 50-250 torr.  Input microwave power: 2.7 kW.  Discharge power density: 80-200 W/cm .  Substrate temperature: maintained equal to 900 ° C. 3 The growth temperature was measured by a MIKRON M675 infrared pyrometer.  Growth rate: 1-20 µm/h. Figure 2.31 – Experimental setup: (1) cylindrical cavity, (2) coaxial waveguide, (3) rectangular waveguide, (4) circulator with a match load microwave to absorb reflected microwave power, (5) magnetron, (6) microwave discharge, (7) quartz cell, (8) buffer vacuum volume, (9) pump out system, (10) gas-feed system, (11) magnetron power supply, (12) control PC, (13) diagnostic window, (14) SOLAR MS 3504 monochromator, (15) photomultiplier, (16) digital oscilloscope, (17) PC, (18) monochromator controller, (19) flat substrate holder, and (20) holder in the shape of truncated cone (trapezoid holder) [59]. 63 High MWPD was obtained by working at relatively high gas pressures or by using local increase of the electric field by changing the substrate holder configuration. The two substrate holders, i.e. item (19) and (20) in Figure 2.3.1 ensure different configurations of the microwave field near the diamond substrate. The dependences of MWPD on the pressure for two types of holders is shown in Figure 2.32 [59]. The MWPD was calculated using incident microwave power and discharge volume which was estimated from the discharge photos from different views. The uncertainty in MWPD measurements was about 10%. Figure 2.32 shows that the MWPD is higher for the trapezoid substrate holder than that with the flat one. For instance, pressure 210 torr for flat holder (point D) can achieve the same power density as trapezoid holder at 145 torr (point C). Figure 2.32 – Dependence of the MWPD on the gas pressure for two different 64 configurations of the substrate holder. Letters A, B, C, and D (circles) mark the parameters, for which the comparison of the diamond deposition was made [59]. A series of experiments on the single crystal diamond growth were performed with varying growth parameters described above. Several examples after deposition are shown in Figure 2.33. The sample in Figure 2.33(a) was grown in the condition of point A in Figure 2.32 with 4% methane and 5 µm/h growth rate. Samples in Figure 2.33(b) and (c) were both grown under the condition of point B in Figure 2.32, with growth rates of 1.5 µm/h (4% methane) and 5 µm/h (8% methane), respectively. The growth time and the thickness of diamond film were not provided. However, it seems that the film is not very thick and is still on the top of the HPHT substrate. Figure 2.33 – Photos of the samples after the deposition process [59]. 2.3.4.2 Single crystal diamond growth at continuous and pulsed mode The synthesized SCD deposition was performed under the same conditions as described in previous section, except the pressure was varied 65 from 150 to 260 torr, and substrate holder was the flat holder. The methane content was 4% and 8%. The pulse repetition rates were 150 and 250 Hz. For both frequencies the duty cycle was fixed to 50%. Figure 2.34 shows the dependences of the MWPD on the gas pressure for CW regime (solid lines) [59] and PW regime (dashed lines) [61]. The circles and letters A, B, C, D, E, F and G in the figure show the experimental conditions, at which the single crystal diamond deposition was performed and compared. Figure 2.34 – Dependence of the MWPD on the gas pressure: solid lines – CW mode and dash lines – PW mode [61]. Figure 2.34 shows that the same MWPD values are achieved at higher gas pressures when one passes from CW regime to the PW regime. In PW regime, different pressures can achieve the same MWPD value at different 66 pulse repetition rates. For instance, 250 torr and 260 torr are both at 200 3 W/cm for 250 Hz and 150 Hz, respectively (points E and F in Figure 2.34). The SCD growth experiments with pulse mode were performed under the same conditions as the points shown in Figure 2.34. Figure 2.35 shows the comparison of the diamond growth rates in CW and PW regimes of MPACVD 3 reactor operation for an equal MWPD of 200 W/cm (points C, D, E and F in Figure 2.34). Figure 2.35 – Dependence of the SCD growth rate on the gas pressure for CW 3 and PW regimes at the same MWPD of 200 W/cm and the methane content of 4% (circle) and 8% (triangle). Letters on the graph corresponds to letters in Figure 2.34 [61]. 67 The experimental results show that the growth rate for a methane content of 8% was higher in all cases than that for a methane content of 4%. For the same power density (points C, D, E and F), the diamond growth rate was higher under higher gas pressure, and was higher in PW regime than that in CW regime. At a pulse repetition rate of 150 Hz the high quality diamond was grown with growth rate of 22 µm/h. However, this statement, as well as a comprehensive study of the physical mechanisms which underlie the growth rate increase in the PW regime, requires further analysis. 68 2.3.5 University of Bristol, UK M.N.R. Ashfold et al. [91, 92, 105-107] from University of Bristol collaborated with Y.A. Mankelevich from Moscow State University, Russia and J.E. Butler from Naval Research Laboratory, USA to provide the understanding of CVD diamond growth. Theoretical numerical models of the gas phase chemical and plasma kinetics were built up to provide insight into the distribution of critical chemical species throughout the reactor in the MPACVD diamond synthesis process. They have presented a detailed description of a two-dimensional model of the plasma-chemical activation, transport, and deposition processes occurring in microwave (MW) activated H/C/Ar mixtures. The model results were verified and compared with a range of complementary diagnostic experimental data. The experimental setup is shown in Figure 2.36 [106], which includes a 2.45 GHz MPACVD reactor (Element Six Ltd.). These comparisons included measured (by cavity ring down spectroscopy) C2, CH, and H column densities, and infrared (quantum cascade laser) measurements of C2H2 and CH4 column densities over a wide range of process conditions. The cavity ring down spectroscopy (CRDS) is an ultrasensitive multipass absorption technique, which yields the line-integrated absorbance (LIA) associated with the chosen probe transition with vibrational and rotational quantum state resolutions. This LIA can be converted to an absolute column (i.e. along r) density (species number per area) in the quantum state along the 69 probed column. The species number density can be obtained by dividing the derived column density by the column length. Figure 2.36 – Schematic diagram of the MPACVD reactor illustrating the position of the substrate, plasma ball, and the side arms for probing by CRDS [106]. The 2D model took account of changes in plasma parameters and conditions (e.g. in Tgas, Te, the electron density (ne), the power density and the plasma chemistry) induced by varying reactor parameters like pressure, input power, and the mole fractions of CH4 and Ar in the process gas mixture. Typical values for the plasma parameters in the plasma core returned by the 3 2D model are [92]: Tgas ∼ 2800-2950 K, power densities 20-40 W/cm , reduced electric fields E/N ∼ 25-30 Td, ne ∼ (2-3) × 10 11 cm −3 and H atom mole fraction XH ∼ 8% for the base conditions: pressure = 150 torr, input power = 1.5 kW, CH4 flow rate = 25 sccm, Ar flow rate = 40 sccm and H2 flow 70 rate = 500 sccm, with a substrate diameter ds = 3 cm and temperature Ts = 973 K, a model reactor chamber of diameter dr = 12 cm and height h = 6 cm, and the following external parameters: cylindrical plasma bulk with radius rpl ∼ 2.9 cm and height 0 < z < hpl = 1.4 cm, and Te ∼ 1.28 eV. Figure 2.37 shows 2D (r, z) false-color plots depicting the electron and H atom number density profiles returned for these conditions. The right panel of the figure clearly shows the large fall in [H] at small z (reflecting H atom loss by reactions at the substrate surface along with (see Figure 2.38) the rapid decline in the local Tgas). Figure 2.37 – 2D (r, z) plots of the calculated (left) electron and (right) H atom concentration, for substrate holder diameter = 3 cm and input power = 1.5 kW. The scales on the left and right under the plot belong to two plots of electron and hydrogen atom concentration, respectively [107]. 71 Figure 2.38 shows 2D (r, z) false-color plots depicting variation of Tgas and of the methyl radical density, [CH3], within the MPCVD reactor operating under base conditions described above. Three regions are labeled in the panels of this figure: the central, hot plasma region A, and two hemispherical shells, B and C, characterized by different average Tgas and XH values. The CH4 source gas is converted into C2H2 in region B, at gas temperatures 1400 K < Tgas < 2200 K, leading to local maxima of the CH3 number density in this region. The reverse C2H2 →CH4 conversion dominates in region C, at gas temperatures 500 K < Tgas < 1400 K, with the result that C2H2 mole fractions shows local minima in region C. The identification of regions A-C provided an obvious rationale for the observed insensitivity of the deposition process to the particular choice of hydrocarbon process gas (CH4, C2H2, C2H4, C3H8, etc). Figure 2.38 also serves to illustrate the fact that the chemically reactive region (determined by the H atom activated hydrocarbon chemistry and Tgas) can be considerably larger than the visible glowing plasma (associated with electron impact excitation of species that then decay radiatively). 72 Figure 2.38 – 2D (r, z) plots of the calculated (left) gas temperature, T gas, in Kelvin and (right) CH3 number density, for substrate holder diameter = 3 cm and input power = 1.5 kW. The scales under the plot belong to two plots of concentration, respectively [107]. Absolute column densities of C2(a), CH radicals and H(n=2) atoms was experimentally measured as functions of height (z) above the substrate surface and process conditions. The MPACVD reactor shown in Figure 2.36 was operated with CH4/Ar/H2 gas mixtures for the measurement. In order to compare the experimental data with the 2D model, the “base” experimental conditions were held the same: pressure = 150 torr, input power = 1.5 kW, CH4 flow rate = 25 sccm, Ar flow rate = 40 sccm and H2 flow rate = 500 sccm. Figure 2.39 shows column densities for C2 and CH radicals and for H atoms 73 measured along the column at z = 11 mm above the substrate surface. In all cases, the two radical column densities can be plotted conveniently on a common scale, indicated on the left hand axis of each plot. The H column densities are typically four orders of magnitude smaller—as shown on the right hand vertical axis in each of the plots. When recording the data in Figure 2.39(a) and (b), H2 flow rate was adjusted to ensure that total flow rate was held constant when flow rate of CH4 or Ar varied. Also shown in Figure 2.39 (open symbols) are the values returned by the 2D modeling. Clearly, the modeling succeeds in capturing both the absolute column densities and their variation with process conditions, well. Each panel displayed in Figure 2.39 illustrates the consequence of varying one parameter. Figure 2.39(a) show the effect of varying methane flow rate from zero to 40 or 45 sccm. Adding just 5 sccm of methane to the plasma results in an approximately twofold jump in the H column density, but further addition causes a progressive decrease in this column density. The C2 column density is seen to scale essentially linearly with methane flow rate, whereas the rate of increase in the CH column density declines as methane flow rate increases. Figure 2.39(c) (d) shows that all three column densities increase as pressure or input power are increased. 74 Figure 2.39 – Filled symbols: column densities for C2 and CH radicals (left hand scale) and for H atoms (right hand scale) plotted as functions of (a) CH4 flow rate, (b) Ar flow rate, (c) applied MW power P, and (d) total pressure p. The open symbols show values for the corresponding quantities returned by the 2D model calculations [106]. 75 Figure 2.39 (cont’d) In conclusion, by two-dimensional (r, z) modeling of the plasma chemistry, the H distribution was seen to peak above the substrate, reflecting its sensitivity both to thermal chemistry (which drives the formation of ground 76 state H atoms) and the distributions of electron density (ne) and temperature (Te). All three column densities were found to be sensitively dependent on the C/H ratio in the process gas mixture, input power and operational pressure. The excellent agreement between measured and predicted column densities for all three probed species, under all process conditions investigated, encouraged confidence in the predicted number densities of other of the more abundant radical species adjacent to the growing diamond surface which, in turn, reinforces the view that CH3 radicals were the dominant growth species in microwave activated hydrocarbon/Ar/H2 gas mixtures used in the CVD diamond synthesis for both microcrystalline and single crystal diamond. 2.3.6 Element Six Ltd., UK Element Six Ltd. has been working on synthetic diamond synthesis using MPACVD process since 1989 [109]. Though the specific detail information of the process was not provided, they produced high quality single crystal diamond that was identified as type IIIa diamond. A type IIIa synthetic single crystal diamond plate was purchased from Element Six to be used as a reference diamond sample of excellent quality in this dissertation research. This crystal is grade of optical low absorption plus low birefringence. The nitrogen impurity level in the crystal is around 30 ppb, which was measured by EPR (electron paramagnetic resonance) and absorption measurements. 77 2.3.7 Summary In summary, the most recent MPACVD SCD synthesis technologies of the four groups introduced in the Sections 2.3.1-2.3.4 are summarized in Table 2.5. For each research group, the experimental condition input/output process variables that were employed in their SCD synthesis process are listed in this table. The selected important experimental condition variables include the reactor type, the information of substrate seeds, the substrate pre-treatment method, the basic experimental conditions (gas chemistry/purity, pressure, input power, discharge power density and substrate temperature), and the range of expected growth rate. In Table 2.5, the information in the column of the Carnegie group was summarized from the most recent improved SCD synthesis technology for the fabrication of colorless, multi-carat SCD crystal. This method was employed to synthesis high optical quality SCD at high growth rates without annealing. The experimental conditions shown in the column of the AIST group were used in their recent development of large area SCD plate fabrication process. The SCD synthesis conditions in the research of the RAS group are summarized in the Table 2.5 as well. The LIMHP group focused on study of influence of the deposition process parameters on the SCD growth, thus the experimental conditions in this column are not very specific. However, their work concerned with the SCD synthesis is important for comparison with the research results presented in this dissertation. 78 Table 2.5 – Experimental conditions of recent MPACVD SCD synthesis from various research groups [13, 30-39, 43-48, 58-60]. Carnegie [13] AIST [30-39] LIMHP [43-48] RAS [58-60] Reactor type ASTeX/Seki ASTeX/Seki LIMHP MSU Substrate HPHT type Ib HPHT type Ib HPHT type Ib HPHT type seeds SCD seeds SCD seeds (9 SCD seeds Ib SCD (9 mm x 9 mm x 9 mm) (3.5 mm x 3.5 seeds mm) or CVD SCD mm) (2.5 mm x 2.5 mm) Substrate Ultrasonic pre-treatment cleaning 0.5 h H2 Wet-chemical plasma etch not provided cleaning + 2 h H2/O2 plasma etch Gas 8-22% 12% chemistry CH4/H2 H2, CH4/ 2-7% CH4/ H2, 1-12% CH4/ 0.12% <0.02% N2/ H2 H2 N2/ H2 Gas purity H2: 7N H2: 6N H2: 9N CH4: 5-5N CH4: 6N not provided CH4: 6N N2: 4N Pressure 100-200 torr 160-180 torr 100-200 torr 50-250 torr Input power 3-5 kW 2-3 kW 2-4 kW 2.7 kW 79 Table 2.5 (cont’d) Power 50-100 density W/cm Substrate 1100-1300 ° 1130-1150 ° 800-1000 ° C C C 900 ° C 50-100 µm/h 1-20 µm/h 3 100-150 3 W/cm 60-130 3 W/cm 80-200 W/cm 3 temperature Growth rate 30-50 µm/h 80 5-55 µm/h CHAPTER 3 THE MPACVD REACTOR AND ASSOCIATED SYSTEMS 3.1 Introduction This chapter presents a description of the microwave plasma assisted chemical vapor deposition (MPACVD) reactor, Reactor B, which was used in the experiments reported in this dissertation research. It begins with a summary of the work that was done by previous students using this reactor. The experimental systems, including the MPACVD Reactor B and associated sub-systems, such as the gas flow control and the vacuum systems, are described. The external microwave system and microwave coupling efficiency are also discussed briefly. 81 3.2 Summary of previous work The MPACVD reactor that was used to synthesize single crystal diamond (SCD) in this dissertation research is identified as Reactor B. Compared to the earlier, conventional versions of the reactor (Reactor A) [65-67], Reactor B was designed to provide reliable and safe operation at higher discharge power densities and higher pressures [1, 68]. Figure 3.1 shows a cross section of Reactor A and the major differences between the two reactor designs. Both reactors have a same cylindrical, phi-symmetrical geometry, and identical cavity applicator radii, R1 and R2, but have different substrate holder / cooling stage radii, R3 and R4. The details of the Reactor B design are provided in later section. 82 Figure 3.1 – (a) Cross sectional view of the Reactor A, (b) the modified 83 substrate holder with reduced inner conductor (cooling stage) radius of Reactor B. The z = 0 plane separates the cylindrical and coaxial sections of the reactor [68]. The other dimensions of Reactor B are the same as Reactor A. 3.2.1 Reactor A for polycrystalline diamond deposition The conventional Reactor A has been operated for polycrystalline diamond thin film deposition for over 20 years [65-67]. Using the 2.45 GHz microwave cavity applicator, K.P. Kuo et al. experimentally investigated the diamond film deposition on Si substrates at pressures of 80-140 torr [66, 76]. In order to evaluate the performance of the reactor for diamond deposition, the experimental variable space was introduced. They were arranged into three basic groups: (1) input variables, such as reactor geometry, substrate, deposition time, pressure, microwave power, gas chemistry, and substrate temperature, etc. (2) internal variables, such as discharge volume, absorbed power density, etc. (3) output variables, such as growth rate, film uniformity, morphology and quality of diamond film, etc. Numerous experiments were performed by varying the input variables. In these experiments, the substrates were 50.8 mm diameter Si wafers. In particular, diamond film growth rate (weight gain: mg/h) versus methane concentration, and substrate temperature were investigated. Figure 3.2 displays the variation in diamond film growth rate as the methane concentration was independently varied from 1 to 8%. During each 84 experiment, the pressure, flow rate, substrate temperature and deposition time were held constant at 135 torr, 600 sccm, 1058 ° and 5 h, respectively. The C, results indicated that the average film growth rate increased as methane concentration increased, and achieved the highest value of 4.3 µm/h at methane concentration of 3%. Then the growth rate decreased for higher methane concentrations. Figure 3.3 displays the growth rate versus the substrate temperature. Pressure, flow rate, methane concentration and deposition time were held constant at 135 torr, 618 sccm, 3% and 10 h, respectively. The diamond film growth rate varied as the substrate temperature was increased from 950 to 1128 ° The maximum growth rate occurs at 1100 ° C. C. In summary, at pressures of 80-140 torr, the microwave absorbed power 3 density of Reactor A increases to 20-30 W/cm . For the optimum growth conditions, the methane concentration, flow rate, substrate temperature, pressure and absorbed power density should be held in the rage of 3-4%, 3 600-700 sccm, 1060-1100 ° 120-135 torr, and 30 W/cm , respectively. Under C, these conditions the uniform polycrystalline diamond film deposition rates could increase to 4.6-7 µm/h. 85 Figure 3.2 – The growth rate (total weight gain and linear) versus methane concentration [66]. (Experimental condition: 135 torr, 600 sccm, 1058 ° 5 h, C, 3 22.5 W/cm ) Figure 3.3 – The growth rate (total weight gain and linear) versus substrate 86 temperature [66]. (Experimental condition: 135 torr, 618 sccm, 3%, 10 h, 21.4 3 W/cm ) S.S. Zuo et al. continued the investigation of polycrystalline diamond synthesis using Reactor A [67]. Over a pressure regime of 60-180 torr, the hydrogen flow rate was fixed at 400 sccm, and methane concentration was varied in the range of 1-2%. The deposition substrates were Si wafers with diameters of 50 mm and 75 mm and thickness of 1 mm. A goal of Zuo’s research was to deposit uniform, thick, high quality polycrystalline diamond films over large substrate surface. In order to achieve uniform growth of diamond film over a deposition area with diameter up to 75 mm, it is important to achieve good temperature uniformity across the substrate, especially for high pressures greater than 120 torr. With the internal microwave circuit tuning structure of the reactor design, the discharge was adjusted to extend across the substrate with good contact between the plasma and substrate. The range of operating conditions for coverage of a 75 mm diameter substrate was experimentally determined. As shown in figure 3.4, substrate center temperature is a function of pressure and microwave power with methane concentration of 1%. The vertical bars associated with the data points represent the minimum/maximum temperature variation across a 75 mm diameter substrate. The dotted-line parallelogram provides useful information concerning the approximate operation range for 87 discharge coverage of substrate area with a diameter of 75 mm. The left hand side of the parallelogram indicates the minimum operational power which is required to generate a discharge of sufficient size to cover the substrate. The right hand side of the parallelogram represents the maximum power limitation at which the discharge size becomes large enough to approach the walls of the bell jar. Figure 3.4 – Operating field map for Reactor A, i.e. substrate center temperature versus pressure and absorbed microwave power for the deposition plasma [67]. Figure 3.4 also shows that non-uniformity of substrate temperature is relatively worse at higher pressures than that at lower pressures. It can be compensated by using a water-cooling substrate to preferentially remove heat from the central portion, and by adding argon to the deposition gases (1/2 argon/hydrogen). The addition of argon increased the plasma size, thus 88 helped to achieve deposition uniformity. Thickness uniformities of ± 5% across 75 mm diameters were achieved with growth rates of 1.9 µm/h and with methane concentrations of 2% at a pressure of 100 torr. Figure 3.5 shows a diamond sample after silicon removal. The diameter was 50 mm and thickness is 75 µm. After further post-processing steps, smooth and uniformly thick films were fabricated for various technical applications. Figure 3.6 shows an example of a diamond window with thickness of 35 µm, which was laser cut, lapped, polished, and mounted in a fixture for an ion beam electron stripper application. In summary, the polycrystalline CVD diamond deposition was operated by Reactor A with pressure up to 180 torr and over area of 75 mm diameter. The reactor operation regime was investigated and determined. Flat, smooth and uniformly thick diamond films were produced. 89 Figure 3.5 – A free-standing polycrystalline diamond sample, 75 µm thick [67]. Figure 3.6 – A lapped and polished polycrystalline diamond sample, 35 µm thick, mounted for an ion beam electron stripping application. The window opening is 1 cm x 1 cm [67]. 90 3.2.2 Reactor B for polycrystalline diamond deposition In order to allow operation at higher discharge power densities and at pressures greater than 180 torr, the design of Reactor A was modified [1, 68]. As shown in Figure 3.1, compared to Reactor A, the substrate holder radius R4 was reduced from 5.08 cm to 3.24 cm, and the inner conductor cooling stage radius R3 was reduced from 4.13 cm to 1.91 cm. The redesign also involved allowing the substrate holder stage to be tunable, i.e. allowing L1 and L2 to be variable. This modification focuses the electromagnetic energy on the reduced diameter substrate holder and thus increases the discharge power densities. The modified reactor design was identified as Reactor B [78]. The performance of Reactor B was evaluated by synthesizing polycrystalline diamond at high pressures up to 240 torr [68]. K.W. Hemawan et al. used Si wafers with diameter of 2.54 cm as substrates, and also explored the reactor operation range in the similar way as was shown in Figure 3.4 for Reactor A. That is how the reactor operating field map was constructed. For Reactor B, Figure 3.7 shows the substrate temperature versus absorbed microwave power for pressures of 60-240 torr, at a methane concentration of 3% and substrate holder position of -0.31 cm. Similarly, the dashed line enclosed area is the safe and efficient experimentally operating range for the coverage of a 2.54 cm diameter substrate area. 91 Figure 3.7 – Substrate temperature versus absorbed microwave power at various operating pressures [68]. The reactor operation was further optimized by adjusting the substrate holder position. The position of the substrate holder defined as Z s = L1-L2, varies approximately between +0.5 cm to about -0.5 cm around the z = 0 plane (in Figure 3.1). An example of an optimization process is displayed in Figure 3.8. For pressure of 220 torr and methane concentration of 3%, a set of eight-hour deposition experiments were performed that explored the diamond growth rate and substrate temperature variation versus substrate position Z s. 92 Figure 3.8 – (a) substrate temperature and (b) diamond growth rate versus substrate position [68]. Figure 3.8 shows that small adjustments of Zs had an important influence on the growth rates and substrate temperature. By adjusting the substrate position from positive to negative, the growth rate and substrate temperature both increase. These experimental results demonstrate the need 93 at the high pressures to vary the reactor’s coaxial cavity dimensions to achieve optimum diamond synthesis, i.e. uniform and high deposition rate. Using the optimized Reactor B, a group of polycrystalline diamond deposition experiments were performed with pressures varying from 180 to 240 torr and methane concentrations varying from 2% to 5%. Figure 3.9 shows that the growth rates increased as methane concentrations were increased from 2% to 5%, and increased as the pressure was increased from 180 to 240 torr. For similar methane concentration, the polycrystalline diamond growth rates at higher pressures by Reactor B are two to three times higher than the growth rates obtained at lower pressures by Reactor A [66, 67]. Figure 3.9 – Diamond growth rate with increasing operating pressure with methane concentration ranging from 2 to 5% with no addition of nitrogen gas into the system [68]. The diamond films deposited at high pressures also have good quality. 94 The photograph shown in Figure 3.10 displays a typical free-standing 25.4 mm diameter, 70 µm thick diamond film. It was lapped, polished and removed from substrate after deposition at 200 torr with a 2% methane concentration. Figure 3.10 – An example of free-standing diamond film adjacent to a quarter dollar coin, after being polished, lapped and Si substrate removal via chemical etching [68]. In summary, the Reactor B design was an evolution from the Reactor A design. Two major changes were made (1) by reducing the substrate holder area and (2) by allowing the substrate holder position to vary about z = 0 plane. Reactor B was experimentally evaluated by depositing polycrystalline diamond over 180-240 torr pressure regime. The reduction of the inner conductor produced very intense discharges at high pressures. The variation of Z s about z = 0 plane allowed the deposition rate to be optimized. The deposition experiments showed that diamond growth rates increased with increasing 95 operating pressure and higher methane concentration, and were two to three times higher than growth rates obtained by Reactor A. Thick, free-standing diamond films were synthesized and have high optical quality. 3.2.3 Reactor B for single crystal diamond synthesis Based on the previous work of polycrystalline diamond deposition summarized above and initial results of single crystal diamond (SCD) synthesis [1], this dissertation research was focused on the investigation of high quality, high growth rate SCD synthesis by Reactor B at high pressures and high power densities. The performance of the reactor at high pressures up to 280 torr was further evaluated. An extended operational map for Reactor B, i.e. the experimental measurement of substrate temperature versus on pressures and absorbed microwave power, was experimentally determined, and the efficient and safe experimental operating regime was defined. By exploring the experimental variable space, the influence of input experimental variables on MPACVD SCD synthesis was investigated. The process control methods of the reactor operation for SCD synthesis at high pressures were also introduced. 96 3.3 The microwave plasma assisted chemical vapor deposition experimental system This section describes the MPACVD experimental system for SCD synthesis at high pressures. The system setup involves not only the microwave plasma reactor, but also its associated subsystems, and many additional components. The overall experimental setup is showed in Figure 3.11. The MPACVD experimental system consists of the following subsystems: (1) the microwave plasma reactor, (2) the microwave power supply with associated microwave transmission network, (3) the gas flow control, (4) the vacuum pumping system and pressure control, and (5) additional components such as computer control, water cooling chiller and air cooling fans. 97 Figure 3.11 – Overall setup of MPACVD Reactor B experimental system. 3.3.1 Introduction of microwave plasma reactor The microwave plasma reactor, Reactor B, which was used in this dissertation research, was optimized to operate at pressures from 180 torr to 280 torr. As described in the section above, Reactor B is a modification of an earlier conventional reactor design [65-67, 69], Reactor A. Figure 3.12 shows 98 the cross section of Reactor B. Figure 3.12 – Cross section of microwave plasma Reactor B. A brief introduction of Reactor B is summarized as follows.  Compared to Reactor A, Reactor B was redesigned to allow operation at higher discharge power densities and higher pressures. This was done by reducing the substrate holder radius R4 from 5.08 cm to 3.24 cm and the coaxial cavity inner conductor radius R3 from 4.13 cm to 1.91 cm. The coaxial cavity center conductor area was reduced by a factor of about 4.5 [1, 68]. The cavity radius, R1 and the quartz dome 99 radius, R2 were not changed and were identical with the radii of Reactor A. R1 and R2 are 8.89 cm and 7.04 cm, respectively.  The applicator consists of two resonant waveguide sections that behave approximately as two coupled cavities, i.e. a cylindrical cavity section (z > 0) and a coaxial cavity section (z < 0). The two cavities are coupled at the z = 0 plane which is also the cylindrical cavity bottom plane. The applicator is excited in a hybrid TM013 (in the cylindrical section) + TEM001 (in the coaxial section) electromagnetic mode. In order to achieve the hybrid excitation, the top (z > 0) cylindrical section of length Ls and the coaxial section (z < 0) of length L2 have to be adjusted to the proper lengths.  There is a sliding short and an excitation coupling probe, both of which are tunable for system microwave matching and process optimization. In this research, the length of probe, Lp, was usually kept constant, about 3.56 cm. The length of the short, Ls, had to be adjusted as L2 was varied.  The length of the coaxial section (z < 0), L2, was controlled and altered by adding circular stainless shims of various thicknesses (in Figure 3.13). As is shown in Figure 3.13, L1 is defined as the distance between the top of substrate holder and the bottom of the coaxial section. As the substrate holder design varies due to the variation in the molybdenum 100 holder thicknesses, L1 is also adjustable. The configuration details of cooling stage and substrate holders are described in a later section.  The varying of geometric length variables, L1, L2, Ls and Lp, results in change of the electromagnetic fields in the local region above and around z = 0 plane, and also alteration of the plasma shape, position and discharge power density. In practice, an optimal relation of L1 and L2 is L1 < L2, and L2 ≈ λ0/2, where λ0 is the TEM free space electromagnetic wave length, 6.12 cm for 2.45 GHz excitation frequency. Figure 3.13 – Detailed cross section of the inner conductor water cooling stage, substrate holder, and shims configurations. Units are in inches [1]. 101 3.3.2 Other subsystems of the MPACVD experimental system 3.3.2.1 Microwave power supply and circuit transmission network Figure 3.14 displays the microwave power supply and the microwave transmission circuit. The 2.45 GHz microwave energy is supplied by a 6 kW Cober power source (model: S6F/4503). The magnetron tube and the circulator are water cooled inside the power box. The incident power is transmitted through multiple sets of S band waveguides in the following order: (1) power supply, (2) a straight copper waveguide (WR340), (3) a 90°E bend waveguide, (4) a flexible non-twistable straight waveguide, (5) a waveguide adapter transition unit to a coaxial waveguide section, and (6) the coupling probe of the microwave plasma cavity. The (7) incident power meter, (8) reflected power meter and (9) dual directional couplers (attenuation factor 60 dB) are installed along the straight waveguide to measure the incident and reflected power in the cavity. The absorbed microwave power (Pabs) is defined as the difference between the incident power (Pinc) and reflected power (Pref). The external microwave system and microwave coupling efficiency is discussed in section 3.5. 102 Figure 3.14 – Microwave power supply and waveguide network subsystem [1]. 3.3.2.2 Gas flow control The input gas sources consist of methane (CH4), argon (Ar), hydrogen (H2), carbon dioxide (CO2) and nitrogen (N2). The H2 and CH4 input gases had purity levels of 99.9995% (5.5N) and 99.999% (5N) respectively. The N 2 103 input gas was 0.9255% nitrogen balanced by hydrogen, with an analytical uncertainty of ± 2% of the nitrogen concentration. The input gas flows were monitored by a four channel MKS mass flow controller (model 247C). CO2 and N2 shared one channel as they were not used simultaneously. The input gases were mixed before they enter the discharge chamber and the gas flowed out through 8 equally spaced circular holes which were cut into the conducting bottom plate of cooling stage (see Figures 3.12 and 3.15). 3.3.2.3 Vacuum pumping and pressure control A mechanical roughing pump (Adixen 2021SD) was used to pump down the chamber pressure to several milli-torr (mtorr). The chamber pressure was measured by a pressure gauge (MKS model 627B). The operation pressure was controlled by a pressure controller (MKS model 651) which was also connected to an automatic throttle valve. As shown in Fig. 3.11, there is a system vent valve that was used to back fill nitrogen into the vacuum chamber before the chamber was opened to atmosphere. For the gas exhaust section, there is also a gas line for nitrogen purging purpose during the experiment. This ensured that the exhaust gas mixtures coming from the output of the roughing pump was below the flammable limit. 104 3.3.2.4 Additional components The control of the input gas flows, the operating pressure and the microwave input power were all synchronized with a Lab-View computer module. It was used to automatically monitor and regulate the system during the diamond deposition experiments from start up to shut down. During the experiments, the operating pressure, gas flows, and running time were all controlled by the computer based on the preset values. If for some reason, the reflected power was more than 25% of input power, or if the pressure fluctuations were more than 1 torr, the computer would automatically shut down the microwave power supply. The cooling system consisted of a water-cooling chiller, an air blower, and four air cooling fans. A water chiller (Neslab CFT-300) provides water-cooling for the substrate holder, cavity base plates and cavity top plates at a set temperature, which could be varied from 15 ° to 22 ° during the C C experiment. For air cooling, there were usually three air fans placed around the cavity to cool the cavity wall from different directions, and a fan was also placed beside the power supply box to cool the power supply from outside, when the synthesis process is running. An air blower was used to cool the quartz bell jar through an inlet of the cavity wall [77]. 105 3.4 The cooling stage and substrate holder configurations 3.4.1 Cooling stage The cross section of the cooling stage that was used for diamond synthesis in this research is shown in Figure 3.15. This cooling stage is made out of stainless steel. The cylindrical body outer diameter is 3.81 cm (1.5 inches) and the inner diameter is 3.49 cm (1.375 inches). The height of the stainless tube is 5.08 cm (2 inches). At the bottom center of the stage, there are input and output water openings with 0.95 cm (0.375 inch) diameter. The opening which is closer to the center is for water to flow in, and the one further away from the center is for water to flow out. The diameter of the conducting bottom plate is 15.24 cm (6 inches). There are 8 equally spaced circular holes with 0.635 cm (0.25 inch) diameter in the base plate of the stage, as shown in the drawing of the cross section in Figure 3.15. The openings are in the bottom plate of the stage. The output gas flows through these openings. In Figure 3.15, there is a circular groove close to the cylinder body on the bottom plate of the cooling stage. The groove is designed for placing a quartz tube underneath the substrate holder. The tube helps prevent microwave power concentrating in the middle of the cooling stage body and also reduces microwave discharge breakdown and formation in this region. The configuration of the tube is shown in Figure 3.16. As quartz is going to expand in the high temperature processing environment, there is 1-2 mm 106 spacing between substrate holder and the top of the tube to avoid the contact after the expansion of the quartz tube during operation. Otherwise when the quartz tube expanded to contact the substrate holder, it might be possible that the substrate holder was pushed up and then loosed good contact with the cooling stage. This is undesirable during operation. The spacing of 1-2 mm between substrate holder and the top of the quartz tube was verified to be sufficient for experiments in this dissertation research. Figure 3.15 – Drawing of cooling stage inner conductor cross sections. Units are in inches [1]. 107 Figure 3.16 – Configuration of quartz tube. 3.4.2 Substrate holder For SCD deposition experiments, there are two types of substrate holder designs that were used in this research. The first type of substrate holder only consists of one piece, which is shown in Figure 3.17. The second type of substrate holder consists of two pieces: the conventional substrate holder for polycrystalline diamond deposition (in Figure 3.18) as the bottom piece, and a insert piece as the top piece (in Figure 3.19). They were both designed with a “pocket” on the top surface where the SCD seeds were placed. Thus they are identified as “pocket holders”. The substrate holders are all made of molybdenum. Figure 3.17 shows the “one-piece” substrate holder configuration. A single piece of SCD seed can be placed in the center pocket with a recess depth of 1 mm to ensure that the 108 seed sits in place during operation. There is an array of 16 holes at the edge of the molybdenum piece. These holes allow the process gases from reactor process to exhaust out of the process chamber into the vacuum chamber, and then out through roughing exhaust gas pump. The substrate holder drawing schematic for polycrystalline diamond deposition is shown in Figure 3.18. The inside diameters is 3.87 cm (1.525 inches) with a thickness of 0.57 cm (0.225 inch). Thus only smaller Si wafers can be used as deposition substrates using this cooling stage, such as 2.54 cm (1 inch) diameter wafers. This holder was used with a 2.54 cm diameter Si wafer as substrate in the experiments that were performed for field map measurements. The details of these measurements are discussed in Chapter 5. For SCD deposition, the holder shown in Figure 3.18 had to be used with a top piece like the one shown in Figure 3.19. The top piece was also made of molybdenum with a diameter of 3.81 cm (1.5 inch). The top piece placed in the opening that was designed for silicon wafer and there is also a pocket design in the center of the top piece. For “two-piece” holders, when the design was altered to optimize the operation, only the top piece was needed. Figure 3.20 shows an example of the top view and side view of a top piece. The details of the configurations of all substrate holders used in this research are summarized in Appendix A. 109 Figure 3.17 – Drawings of “one-piece” substrate holder for SCD synthesis. Units are in inches [1]. 110 Figure 3.18 – Drawings of substrate holder for polycrystalline diamond deposition (or bottom piece of “two-piece” substrate holder for SCD deposition). Units are in inches [1]. 111 Figure 3.19 – 3D top view of the top piece of “two-piece” substrate holder for SCD synthesis. Figure 3.20 – Drawings of the top piece of “two-piece” substrate holder for SCD synthesis (Side view and top view, units in mm). 112 3.5 External microwave coupling system and microwave coupling efficiencies A typical commercially available external experimental microwave plasma reactor system is shown in Figure 3.21. It consists of a microwave power supply, circulator and a matched dummy load, incident and reflected power measuring meters, and the matching system such as tuning stubs and a sliding short, and a microwave plasma loaded reactor. The coupling circuit includes a matching network, which often utilizes external tuning stubs, sliding shorts and waveguides, and the reactor region consisting of the microwave applicator and the discharge. Microwave power is matched at the input plane of the microwave coupling system, which consists of the plasma reactor plus the matching network. The input power delivered to the input plane is P t. Pt is related to the experimentally measured incident power, P inc, the reflected power, Pref, and the power absorbed by the microwave discharge P abs, by the following relationship Pt = Pinc - Pref = Ploss + Pabs. The Ploss term represents the microwave power absorbed by the conducting walls and the dielectric materials that are located between the matching input plane and the discharge. There are two important microwave power losses present in the microwave coupling system: (1) power unused and lost, i.e. reflected power, P ref, that is absorbed in the matched load attached to the circulator, and (2) power losses in the metal walls and in any lossy dielectric materials located to the right of the 113 input matching plane shown in Figure 3.21. Figure 3.21 – External microwave system employed for MPACVD diamond synthesis [79]. In general, the power absorbed by the microwave discharge is given by Pabs = Pinc - Pref - Ploss. The overall microwave coupling efficiency to the discharge, Effcoup, is given by the following equation [65, 70-73], Effcoup = 1 (Ploss+ Pref)/Pinc × 100%. In an optimally designed and operated microwave plasma processing system there is little if any reflected power and there also are low ohmic wall and dielectric losses within the microwave coupling network. Under these conditions the microwave coupling efficiency is very high. However in the typical MPACVD diamond synthesis system the matching network is often located external to the reactor and is many standing wavelengths from the discharge. Thus in some commercial MPACVD reactor 114 systems there may be significant waveguide wall losses, i.e. Ploss may be significant, and the coupling efficiency may be less than 80%. An ideal external microwave system is shown in Figure 3.22. In this figure, the microwave power supply, circulator, and power measurement systems are located very close to the input matching plane. The external microwave system of Reactor B is not quite like the one shown in Figure 3.22 (see Figure 3.14). However, the design of Reactor B minimizes the coupling wall and dielectric losses, since the reactor is “internally matched / tuned”, and the surface areas of the coupling circuit metal walls between the matching input plane and the discharge are minimized. It can be operated in such a way that there is essentially very low reflected power (< 50 W) from the cavity applicator input plane [71-75]. Under all experimental conditions presented in this dissertation the reactor was excited in an understood electromagnetic mode and is operated in a matched condition where little or no power is reflected from the z = Ls input plane (See Figure 3.12). Then there are no standing electromagnetic waves in the coaxial and rectangular coupling waveguides external to the reactor. Standing electromagnetic waves only exist in the short cylindrical cavity applicator sections, i.e. between z = Ls to z = -L2. This enables high coupling efficiencies. Earlier microwave plasma assisted reactor experimental investigations that excited and matched a well-established, plasma loaded, electromagnetic mode inside a MPACVD reactor demonstrated high coupling efficiencies (i.e. > 115 95% and usually > than 98%) under both low and high pressure discharge coupling conditions [65, 70-73, 75]. The experiments described in this dissertation were performed under similar matched reactor conditions and excessive reactor wall heating has not been observed. When the reactor is matched in this manner, i.e. internally matched, microwave coupling to the high pressure discharge is high. Figure 3.22 – The external microwave system used with the MPACVD. Note that reactor matching takes place at the input plane of the cavity applicator which is just one to one and a half wavelengths away from the discharge [79]. 116 In our operating field map measurements and discharge power density calculations it is assumed that Ploss is usually < 2% of Pinc, and hence Ploss has been neglected in the data presented in Chapter 5 and 7, i.e. Pt Pabs for the absorbed power density calculations. More specifically all the experimental and power density data displayed in this dissertation were obtained under well matched conditions, i.e. Pref < 5% of Pinc, and thus for simplicity in the power density calculations Pref and Ploss have been assumed to be zero. 117 CHAPTER 4 THE MPACVD REACTOR OPERATION AND DIAMOND SYNTHESIS PROCEDURES 4.1 Introduction This chapter presents a description of the procedures that were employed when using the microwave plasma assisted chemical vapor deposition (MPACVD) reactor as it synthesized diamond. First a description of the substrate seeds that were used in the experiments is presented. The pre-treatments and post-treatments of the samples and the details of the experimental system’s operational procedures are also described. Then methods of characterization of the diamond samples are also introduced. In the end, the multi-dimensional experimental variable space is summarized. This multi-dimensional experimental variable space is important for the understanding of the reactor’s experimental performance and how to operate the reactor during diamond synthesis process. 118 4.2 Substrates for diamond deposition The experiments of diamond deposition presented in this dissertation consist of polycrystalline diamond deposition and single crystal diamond (SCD) deposition. The polycrystalline diamond deposition experiments were carried out for the purpose of defining the reactor field map. The substrates used in the measurement were 2.54 cm diameter silicon wafers. For SCD deposition experiments, the substrates were high pressure high temperature (HPHT) single crystal diamond seeds. 4.2.1 Substrates for polycrystalline diamond deposition Although the major objective of this dissertation research was to investigate the SCD growth at high pressures, the experiments for Reactor B field map measurements were performed using polycrystalline diamond deposition techniques. The silicon wafer with a 2.54 cm diameter worked well as a reference for the calculation of the volume of discharge. The measurement results could also then be compared with the field maps that were measured for the earlier polycrystalline diamond growth experiments [1, 76, 77]. The silicon wafer that was used as the substrate in the polycrystalline diamond deposition has the following specifications: 2.54 cm diameter (one 119 inch), 1.5 mm thick, N-type, <100> surface orientation, and one side polished. Before the substrate was loaded in the reactor chamber, the silicon wafer was seeded to accelerate the nucleation process of diamond [80, 81]. The seeding method used in this research was mechanically scratching the surface of silicon wafer with diamond power [1, 77, 82, 83]. More specifically, the process began with sprinkling a proper amount of diamond powder on the polished side of a silicon wafer which was clean. The wafer’s surface was scratched by manually moving the diamond powder both in circles and in straight lines in all directions for 10 to 15 minutes, using fingers with a paper cloth. After scratching, the excess diamond powder was removed. The scratches on the wafer surface could be observed by the naked eye if the process had been performed properly, and the scratches should be distributed on the surface evenly and without particularly deep marks. The nucleation density was measured using an optical microscope; however it was not an important goal of this dissertation research. The diamond powder used in the seeding process was 0.25 µm size natural diamond powder. 4.2.2 Substrates for single crystal diamond deposition The substrate seeds that were used in this work are synthetic high pressure high temperature (HPHT) single crystal diamond substrates from Sumitomo Electric Industries (product number: UP353514 and UP353517). 120 They were cut from large high quality single crystal diamonds which were synthesized under ultra-high pressure and temperature conditions. Figure 4.1 shows the shape of a HPHT diamond seed. All six sides of the substrates are (100) face orientation. For most SCD deposition experiments presented in this dissertation, the dimensions of the HPHT diamond seeds that were used are 3.5 mm x 3.5 mm x 1.4 mm (UP353514). However there were a few experiments that used HPHT diamond seeds with dimensions of 3.5 mm x 3.5 mm x 1.7 mm (UP353517). As a matter of fact, the dimensions of the seeds were not exactly as same as they claimed. They varied from seed to seed. The specifications of the HPHT diamond seed are summarized in Table 4.1. Figure 4.1 – The illustration of the shape of a HPHT diamond seed from Sumitomo [84]. As shown in Table 4.1, the thickness (T in Figure 4.1 and Table 4.1) of the seeds were in the range of 1.3-1.5 mm for the seeds with dimensions of 3.5 mm x 3.5 mm x 1.4mm (UP353514). Since the thickness of a seed is a very important dimension for calculation of diamond growth rate, it was measured carefully before and after each experiment. 121 Table 4.1 – Specifications of the HPHT diamond seeds from Sumitomo [84]. Product No. L (mm) W (mm) C1, C2 (mm) T (mm) UP353514 3.5~4.0 3.5~4.0 ~0.4 1.4 ±0.1 UP353517 3.5~4.0 3.5~4.0 ~0.4 1.7 ± 0.1 The two biggest surfaces of the HPHT seeds were mechanically polished, and the other four faces were not. Figure 4.2 shows a typical side view of a HPHT diamond seed. It is one of the sides which were not polished. As shown in this figure, there is difference between dimensions of the top and bottom surfaces. The surface with bigger area was designated as “top surface”, and the other surface was designated as the “bottom surface”. Besides the difference of area, the two surfaces had different growth sector patterns [52]. Pre-treatments which revealed the growth sectors of the seeds are described in Chapter 7. In practice, the top surface was used as deposition surface in experiments and the seed was always placed face up in the pocket holder. Figure 4.3 shows a picture of the top surface of a typical HPHT diamond seed. It is one of the polished faces. The bottom surface has similar morphology, but had a bit smaller area than the top surface. As Figures 4.2 and 4.3 shown, the HPHT seeds have golden-yellow color, because the diamond contains 10-100 ppm nitrogen in the crystal [84]. They are classified as type Ib diamond [84]. 122 Figure 4.2 – Optical micrograph of the side view of a HPHT seed (25x magnification). Figure 4.3 – Optical micrograph of the top view of a HPHT diamond seed (25x magnification). 123 4.2.3 Single crystal diamond pre-deposition cleaning procedure Before each deposition experiment, the HPHT single crystal diamond seed was first cleaned by a wet-chemical cleaning procedure. The cleaning process was intended to remove any metallic, graphite and organic material contamination. The cleaning procedure is described as follows:  Acidic cleaning o Nitric acid (40 mL) + Sulfuric acid (40 mL) mixed in a pyrex beaker, placed on the heater (set to 500 ° for 30 minutes. Then, C) rinse the sample in DI water. o Hydrochloric acid (40 mL) in a pyrex beaker, place on the heater (set to 500 ° for 15 minutes. Then, rinse the sample in DI C) water. o Ammonium hydroxide (40 mL) in pyrex beaker, placed on the heater (set to 500 ° for 15 minutes. Then, rinse the sample in C) DI water.  Ultrasonic cleaning o Ultrasonic bath cleaning with Acetone (30 mL) in a pyrex beaker for 15 minutes. o Ultrasonic bath cleaning with Methanol (30 mL) in a pyrex beaker for 15 minutes.  Final rinsing and drying o Rinse sample with DI water. 124 o Blow nitrogen on the sample to remove water. Once the cleaning procedure was completed, the substrate seed was placed on the substrate holder and loaded in the reactor chamber. The SCD pre-deposition cleaning process should be done in an appropriate laboratory facility, either in or outside a clean room. The acidic cleaning steps must be carried out in a fume hood following relevant operational instructions. The temperature of the heater was adjustable, as long as the acid was boiled. 4.2.4 Single crystal diamond post-deposition cleaning procedure After the deposition was finished and the SCD sample was taken out from the reactor chamber, it was cleaned following the same wet-chemical cleaning procedure which was described above. In order to completely remove the carbon residue on the diamond sample, the first step of acidic cleaning (Nitric acid and Sulfuric acid mixture) was repeated a few times until the sample was clean. Once the post-deposition cleaning procedure was finished, the SCD sample was ready for characterization. Sometimes the grown SCD film was removed from the substrate by laser cutting. Then deposition surface and bottom surface of the CVD diamond sample were mechanically polished and a single crystal CVD diamond plate 125 was fabricated for characterization. The lapping and polishing of the sample was done using a Type PL3 planetary lapping bench (Coborn engineering Co. Ltd.). 126 4.3 The MPACVD experimental system operational procedures In this dissertation research, the MPACVD Reactor B experimental system was run in three types of operational modes: (1) the single crystal diamond deposition experiment, (2) the polycrystalline diamond deposition for Reactor B operational field map measurements, and (3) the plasma etching process for bell jar clean-up. The start up and shut down procedures for the three operational modes were similar. Only a few steps were different. These are described in following sections. 4.3.1 Experiment start up procedure After the substrate preparation procedure, the SCD substrate seed was placed onto the substrate holder (see Figure 3.17-3.20), which was then uploaded and manually fastened onto the reactor chamber. The system was then pumped down, usually over night, to about a few mtorr. The experiment start-up procedure is as follows:  Turn on the main gas inlet valve, and gas regulators.  Turn on cooling water for power supply, chiller, air blower motor, and cooling fans.  Turn on the power supply. It needs a few minutes to warm up. 127  Turn on power meters and gas tank valves, including nitrogen for purging.  Set the operating pressure, gas flow and deposition running time in the stage file in the Lab View module.  Set the MKS pressure controller and gas flow channel to remote position.  Open the prepared stage file from the Lab View module, then the pressure increases as input gas mixture is automatically controlled to fill in the chamber. In this stage, for polycrystalline diamond deposition, the input gas mixture consisted hydrogen and methane; for SCD deposition, the input gas was only hydrogen.  When pressure reaches 5 torr, the microwave power supply is turned on to ignite the plasma discharge. When ignited the discharge, power was set to 0.8 – 1.0 kW.  Slowly increase the input microwave power as the pressure increased.  If necessary, adjust Ls to reduce the reflected power to a minimum.  The deposition process (Hydrogen etching pre-treatment process for SCD synthesis) starts automatically when the pressure reached preset operating pressure value. The input gas chemistry for diamond deposition was varied from experiment to experiment. In SCD deposition experiments, there was a one 128 hour hydrogen-only etching process before the diamond deposition process began. Then methane was introduced into the chamber after the one hour etching process. In the polycrystalline diamond deposition experiments, the methane was mixed with hydrogen from the very beginning of the experiments. The gas mixture chemistry and timing of introduction were preset in the stage file in the Lab View module before the experiments started, thus there was no need for manual control of gas mixture introduction during the start up. 4.3.2 Experiment shut down procedure When the diamond deposition or etching process running time finishes, the system would undergo a shut down procedure as follows:  For SCD deposition, microwave power and methane gas flow are automatically turned off. For polycrystalline diamond deposition, only methane gas flow is automatically turned off. The microwave power and discharge are still on.  The pressure is automatically controlled to slowly drop down to below 1 Torr in a series of steps. In each step the pressure drops 20 torr in a time interval of 3-5 minutes. For polycrystalline diamond deposition, the input microwave power is manually turned down slowly in the process. 129  For polycrystalline diamond deposition, when the pressure reaches 20 torr, the microwave power and hydrogen gas flow are turned off. For SCD deposition, when the pressure is below 1 torr, the hydrogen gas flow is turned off.  Shut down the program, exit the Lab View module, and turn off power meters. Set the pressure controller back to the manual mode and open the valve to the roughing pump. Keep water-cooling and air-cooling system running.  Wait 30 minutes for the system to cool down, before shutting down the power supply.  Turn off chiller, air blower, and all fans.  Turn off gas tanks, gas regulators and the main gas inlet valve.  Wait at least one more hour to ensure safety when opening the vacuum chamber and take the sample out. 4.3.3 MPACVD experimental system cleaning procedure Since there was a carbon coating deposited all over the inside surface of the reactor chamber during the diamond deposition process, the substrate holder, cooling stage and bell jar were cleaned after each diamond deposition experiment. The carbon coating on the inside surface of bell jar was etched away 130 with a microwave discharge plasma consisting of argon and carbon dioxide. The start up and shut down procedures are the same as are described in Section 4.3.1 and 4.3.2. Before the start up of the plasma cleaning process, a floating stage without cooling water connection was uploaded [77]. There was no substrate needed in this cleaning process, and the purging nitrogen was not needed either. The pressure of the cleaning process was set to 60 torr, and microwave power was about 2 kW. The flow rates of argon and carbon dioxide were set to 13 sccm and 7 sccm, respectively. The process running time was 0.5-1 hour. This process was sufficient to remove the carbon coating after a diamond deposition experiment of 8 hours which was the typical experimental run in this work. The carbon residue on the cooling stage, quartz tube, and chamber was removed using tissues soaked with acetone. However there was usually polycrystalline diamond deposited on the substrate holder which was not possible to be removed by tissues. Thus the molybdenum substrate holder pieces had to be sand blasted to get rid of the polycrystalline diamond film. Then the holder pieces were rinsed with DI water to remove sand and any residue. After blowing dry with nitrogen, the pieces were further dried in an oven dryer at about 80 ° for at least 30 minutes. Then the holder was cleaned C and ready for next diamond deposition experiment. The clean-up of the experimental system was important for the diamond deposition and reactor operation. Any impurities existing in the chamber might 131 get into the synthesized SCD diamond. For synthesized SCD, it was one of the major sources for the defects on the diamond sample. The failure of removal of polycrystalline diamond film on the substrate holder might cause an unusual high temperature on the top of the substrate holder which then affected the overall diamond growth conditions. The carbon coating on the bell jar could also have bad influence on the thermal distribution and block the vision of substrate temperature measurement. In order to maintain reliable experimental conditions and grow high quality diamond, the reactor must be kept as clean as possible. The plasma cleaning process for the bell jar might not be helpful if the bell jar was badly coated or burned. Then it must be taken out after disassembling the reactor, and sent to a professional for thorough wet-etching cleaning. 132 4.4 Evaluation of the synthesized single crystal diamond Since the polycrystalline diamond deposition was just performed for the reactor field map measurements, the evaluation of the synthesized diamond in this thesis research only focuses on the synthesized single crystal diamond samples. There were two important diamond synthesis outputs: the growth rate and the quality of grown diamond. 4.4.1 Diamond growth rate calculation Conventionally, the diamond growth rate was calculated by measuring total weight gain [1, 77]. However, compared with the polycrystalline diamond film which hardly grew out of the substrate area, SCD grew much faster and frequently resulted in a grown film with a larger area than the substrate. Thus it is not accurate any more to use the weight gain method for growth rate calculation. For all SCD deposition results presented in this dissertation, in addition to the weight gain method, the diamond film growth rate was also determined by measuring the average film thickness. The SCD film thickness was measured by a linear encoder (Model: Solartron DR600). In this method five measurements points on the top surface of the diamond sample were performed. One point was in the center and the other four points were 133 measured near the corners. Since there were polycrystalline diamond rims on the edges of the sample, the measurements points must avoid the exact corner area. Three measurements were done for each measurement point and the measured thickness of the sample was the average value of all these measurements. The grown diamond film thickness of a particular sample was determined as difference between the average thickness of the sample before and after deposition. The diamond film growth rate was calculated as the film thickness divided by deposition time: Growth rate (µm/h) = Average film thickness (µm) / Deposition time (h). The measurement from the linear encoder was 5 decimal places (unit in mm). The calculated film thickness and growth rate were rounded to 2 decimal places (unit in µm). Only the linear encoder data are reported in this dissertation since they yielded a more precise linear growth rate than weight gain data. 4.4.2 Diamond quality characterization Grown diamond quality was characterized using several methods, such as visual observation of the color and transparency of the samples, optical microscope observations and photos, Raman spectroscopy, ultra violet - infra 134 red (UV-IR) transmission, secondary ion mass spectrometry (SIMS) analysis, and birefringence imaging. 4.4.2.1 Single crystal diamond surface morphology The observation of grown SCD surface morphology was done by using an optical microscope (Nikon Eclipse ME600). The microscope was integrated with a 2-D and 3-D image processing computer program (Image Pro plus 5.1) to allow optical observation and analysis of the samples. By using this instrument, the diamond growth surface smoothness and transparency was identified. The defects, such as twining and stacking faults on the diamond surface or in the crystal, were also observed and analyzed. 4.4.2.2 Raman spectroscopy Raman spectroscopy is one of the most common methods used to characterize the structural quality of grown diamond. This method provides well known spectra for diamond, graphite, and other carbonaceous compounds. By analyzing the diamond peak positions and peak width, Raman spectroscopy was used to determine diamond purity and crystalline lattice matching perfection. Figure 4.4 shows an example of a typical Raman spectrum for a SCD 135 sample grown by MPACVD in Reactor B. The spectra display a sharp peak at 1332 cm -1 3 that indicates sp diamond bonding. Assuming room temperature, the sharp peak of the diamond spectra could shift from 1331 to 1335 with a -1 bandwidth at half intensity usually less than 2 cm . The width of the wavelength range between the two points in the spectrum at which the intensity is equal to half of its peak value is defined as the full width half maximum (FWHM) of the Raman spectrum. The inset of Figure 4.4 displays the close up of the peak area in the Raman spectrum. The red arrow indicates the two points with intensity at the half of the peak value. The width of this -1 arrow is the FWHM of this Raman spectrum, which is 1.6989 cm . The FWHM in natural perfect SCD reported is 1.5-1.8 cm polycrystalline diamond films is between 2 to 14 cm -1 -1 while typical FWHM for [85]. Since there is no apparent difference in the Raman spectra of SCD samples grown in this work, the diamond peak width, i.e. the FWHM was used to evaluate the quality of grown diamond film. A type IIIa diamond sample was compared as a reference. The reference sample was purchased from Element Six as an optical-low absorption plus low birefringence grade crystal. This crystal had a nitrogen impurity level of about 30 ppb and a Raman peak FWHM at 1332 cm -1 -1 of 1.57 cm . It was assumed that the quality of synthesized SCD sample was better if its Raman spectra peak FWHM was closer to the reference Element Six sample. The broadening of the band might be due to several factors such as defects, internal stresses, and the density of 136 impurities in the crystal. Figure 4.4 – Example Raman spectra of a SCD sample synthesized by Reactor B (Sample 80). The inset is the close up of the peak of the spectra. (Experimental condition: 240 torr, 6% CH4/H2) The instrument used in the Raman measurement was a SPEX 1250M spectrometer connected to a HORIBA Jobyn Yvon symphony charged-coupled device (CCD) detector and an Olympus BH-2 optical microscope. The laser source was a 514.5 nm Argon ion green laser with a spot size of 20-30 µm and resolution of 0.2 cm -1 between each acquired data. The spectrometer slit width 137 of 50 µm and the high resolution grating of 1800 grooves/mm were chosen to provide better accuracy. The integration time of 60 seconds and CCD image binning of 1 were set to increase signal/noise ratio and produce high intensity -1 counts. The wavelength could be scanned up to 1800 cm . However at most times, only the wavelength range of 1150-1500 cm -1 was scanned for SCD samples, as the background spectra in the wavelength range of 1500-1800 cm -1 was flat and had no graphite peaks detected. The penetration depth of the laser beam into the sample surface was about a few micrometers, thus the Raman spectra could be considered as a measurement of diamond surface. In order to get accurate measurement, the laser beam must be focused well into the surface. The computer program used to collect and analyze the Raman spectra was Lab Spec 4.01. This program was used to position the diamond peak and determine the FWHM of the Raman spectra using a Gaussian/Lorentzian curve fit and baseline correction. 4.4.2.3 Secondary ion mass spectrometry analysis Secondary ion mass spectrometry (SIMS) is an analytical technique that detects very low concentrations of dopants and impurities. It can provide elemental depth profiles over a depth range from a few angstroms to tens of micrometers. Figure 4.5 shows a schematic diagram of a SIMS analysis system [86]. As shown in the figure, a focused primary ion beam (usually O or 138 Cs) was generated by an ion gun [87]. The sample was sputtered or etched with the ion beam. Secondary ions formed during the sputtering process were extracted and analyzed by using a mass spectrometer (usually a quadrupole or magnetic sector). The secondary ions can range in concentration from matrix levels down to sub-ppm trace levels. SIMS is a good analytical method with excellent detection sensitivity for impurities, with ppm or lower detection sensitivity. Compared to Raman spectra, it can directly detect the impurities concentrations in the diamond crystal and determine the quality of diamond. However, due to high cost of the measurement, only a limited number of SCD samples were analyzed by SIMS. The SIMS analysis results reported in this dissertation was done by Evans Analytical Group [87]. The SCD sample surface was etched by the ion beam and profiled over a depth of 5 µm. The probe size is around 10 µm, and the detection limits was >10 16 3 atom/cm . The specific elements that were chosen to detect were nitrogen (N) and silicon (Si), since they were the most probable impurities existing in diamond deposition environment. Figure 4.6 shows an example of the depth profiling of the concentrations of N and Si in a SCD sample (sample 38). 139 Figure 4.5 – Schematic diagram of a SIMS analysis system [86]. Sample 38 1E+21 Concentration (Atoms/cm3) 1E+20 1E+19 N 1E+18 Si 1E+17 1E+16 1E+15 1E+14 0 1 2 3 Depth (µm) 4 5 Figure 4.6 – Depth profiling of the concentration of N and Si in SCD sample 38 140 (Experimental condition: 240 torr, 5% CH4/H2, 200 ppm N2/H2). As shown in Figure 4.6, the impurities concentrations were measured hundreds times with a depth resolution of about 5 Å. The average value of hundreds of measurements was calculated by using SIMS data processing TM software SIMSview (Evans Analytical Group). Take the data in Figure 4.6 as an example, the calculated densities of N and Si were 7.88 x 10 10 16 17 , and 4.27 x 3 atoms/cm , respectively. Furthermore, the concentration of impurity nitrogen compared to carbon was calculated by using the density of diamond 3 (3.515 g/cm ) and the atomic mass of carbon (12) as follows: N concentration = (7.88 x 10 17 3 3 atoms/cm ) / (3.515 g/ cm x (6.022 x 10 atoms/12g)) ≈ 4.47 x 10 -6 23 = 4.47 ppm. Similarly, the silicon concentration was calculated as 242 ppb. However, due to the detection limit of around 300 ppb, the value of the silicon concentration had no real meaning. It meant that the silicon concentration in the SCD sample was small, well below the detection limit of 300 ppb. 4.4.2.4 Optical transmission measurements The single crystal diamond has a great potential for optical applications as mentioned in Chapter 2. Synthesized CVD diamond should 141 have an excellent optical transparency. In this dissertation, the optical transmission measurement was performed using ultraviolet and visible light with Perkin Elmer Lambda 900 UV/Vis/NIR spectrophotometer. The instrument can measure reflection, absorption or transmission in the range of 180-3300 nm. Figure 4.7 shows the schematic diagram of the instrument. The monochromator holographic gratings were 1440 lines/mm (UV/Vis) and 360 lines/mm (NIR). Figure 4.7 – Schematic diagram of the spectrophotometer. The grown SCD film must be polished and cut from the substrate before optical transmission measurement. It was important to polish and lap the diamond surface in order to obtain excellent transmission in the UV and visible 142 spectrum. The SCD sample was placed in a metal holder which was inserted into a magnetic stage in the system. The polishing and handling process required the grown SCD film to have a thickness of greater than 500 µm. 4.4.2.5 Birefringence imaging Birefringence imaging is an effective method for checking the stress level in crystal. By examining the birefringence patterns of grown SCD sample, the spatial distribution of internal crystal stresses could be obtained. Like optical transmission measurement, the birefringence imaging also required the production of SCD plates with smooth and flat surfaces which should be parallel with each other. In this work, an optical microscope (Nikon Eclipse ME600) was used to take the birefringence images. Two quarter-wavelength retardation filters were positioned at 90°offset rotation. The diamond samples were placed between them. The transmission mode was used in the microscope. In this setup, an unstressed cubic diamond crystal that was optically isotropic would appear black in the image. However, stress and strains in the crystal could make the optical properties anisotropic and cause birefringence, which lead to an observable light intensity distribution in the crystal. Figure 4.8 shows the birefringence image of a typical HPHT diamond seed [88]. The average birefringence intensity was high because of the high impurity concentration in 143 the crystal. The birefringence intensity pattern shown in this figure might be associated with the growth directions during the HPHT diamond production process. Figure 4.8 – Birefringence image of an HPHT diamond seed, 50 ms exposure time [88]. 144 4.5 The multi-dimensional experimental variable space CVD diamond synthesis is a multi-variable experimental problem. There are many experimental variables that have an influence on MPACVD diamond synthesis. Each of these experimental variables was identified and controlled for the MPACVD experimental system used in this dissertation research. They can be arranged into three basic groups [66]: input variables, internal variables and output variables. The entire group of the experimental variables was identified as the multi-dimensional experimental variable space, which contains the important experimental information and represents the reactor’s performance. The experimental variables for Reactor B are listed as following: Input variables  Controllable input variables: o Reactor design: Reactor B. o Reactor matching: Ls and Lp. o Substrate holder design: one-piece or two-piece pocket holder. o Substrate position – Zs: Zs = L1 – L2, optimized and fixed. o Flow rate: ~ 400 sccm. o Growth time: 8-24 hours.  Independently varied input variables: 145 o Absorbed input microwave power – Pabs: Pabs = Pinc – Pref. o Pressure – p. o Nitrogen concentration: 0 – 200 ppm (extra N2 addition). o Methane concentration – CH4/H2. Internal variables o Plasma volume – Vp. 3 o Absorbed power density – W/cm : = Pabs / Vp. o Substrate temperature – Ts. Output variables o Diamond linear growth rate – µm/h. o Diamond quality: Raman. SIMS, UV-IR transmission, and birefringence. As listed above, the input variables were further divided into controllable input variables and independently varied input variables. The group of controllable input variables consists of the variables concerned with the reactor design and geometry, substrate holder setups, and the variables that were not changed for most experiments, such as flow rate and growth time. Once the experimental system was matched and optimized, in most SCD deposition experiments, these variables were kept constant. The variables that represent 146 the experimental conditions, such as pressure, gas chemistry, and microwave input power, belong to the group of independently varied input variables. They were frequently varied for the purpose of investigating the MPACVD diamond synthesis. Figure 4.9 shows a block diagram of these groups of experimental variables. The internal variables, absorbed power density and substrate temperature, etc. are functions of the input variables. They could be observed and measured, but could not be preset. The internal variables could be used to investigate and evaluate the reactor’s behavior. The final output of the process, diamond growth rate and quality, were functions of internal variables, and also function of input variables. One of the major objectives of this dissertation research is to investigate the relationship between the output variables and input/internal variables, i.e. the SCD growth rate and quality versus multi-variable space. Figure 4.9 – Block diagram of the experimental variable groups. 147 CHAPTER 5 EVALUATION OF MPACVD REACTOR B’S PERFORMANCE AT HIGH PRESSURES 5.1 Introduction The microwave plasma assisted chemical vapor deposition (MPACVD) reactor that was used in this experimental thesis investigation of single crystal diamond (SCD) synthesis is identified as Reactor B. Before performing SCD synthesis at high pressures (>180 torr) using Reactor B, the plasma discharge characteristics and Reactor B’s behavior in the high pressure region must be thoroughly, carefully, and experimentally studied. One goal of the experimental research activities was to experimentally determine a safe and efficient experimental operating region for Reactor B. This chapter presents an experimental methodology that defines the safe and efficient experimental reactor operating region. First the experimental conditions and measurement methods are described. This includes a discussion about the photographic techniques and power density calculations. The discharge behavior of Reactor B at high pressures is explored and presented. The operating field map for Reactor B is experimentally determined, and then the safe and efficient experimental operating region is defined. 148 5.2 Experimental techniques The evaluation of the discharge characteristics for Reactor B was done by determining the reactor operating field map for a typical diamond deposition operational condition. It included the measurement of the internal variables of the reactor, such as substrate temperature, microwave power density, etc. and the observation of discharge behavior. The details of the related experimental techniques are described in this section. 5.2.1 The experimental variables that determine the operating field map As described in Chapter 4, the potential experimental operating variable space was vast. The experimental variables could be divided into three groups: input variables, internal variables and output variables. The output variables consisted of the variables that represented the diamond synthesis results. When determining the reactor operating field map, the output variables were ignored. Figure 5.1 displays a block diagram of the experimental variables that were measured when determining the reactor operating filed map. This diagram is evolved from Figure 4.9. As shown in Figure 5.1, the block of output variables is in grey, which indicates that these variables were ignored during the reactor operating field map measurements. Only the important internal variables that described the reactor discharge behavior were the target of 149 measurement and observation. The substrate temperature, Ts, and microwave power density, , were the two measured internal variables. Plasma volume, Vd, was the internal variable that was measured and then was used in the calculation of microwave power density. As already described in Chapter 4 there were many input variables, and there were a huge number of potential input variable operating sets. In order to experimentally determine the safe and efficient operating regime for Reactor B for SCD deposition, the number of input variables was reduced, and a subset of the input variable space was chosen. In the operating field map measurement, most of the input variables were fixed. As shown in Figure 5.1, only pressure (p) and absorbed microwave power (Pabs) were chosen as varied input variables. The target of the measurement was to experimentally determine the variation of substrate temperature and microwave absorbed power density versus pressure and absorbed microwave power for Reactor B. Figure 5.1 – Block diagram of the experimental variables that were measured 150 when determining the reactor operating field map. The fixed input experimental variables are summarized in Table 5.1, and the varied experimental variables are listed in Table 5.2. Table 5.1 – Fixed experimental variables in the reactor operating field map measurement. Fixed experimental variables Reactor design/geometry Reactor B Substrate holder design Polycrystalline diamond holder (Figure 3.18) Substrate Silicon wafer (2.54 cm diameter, 1.5 mm thick) Length of short (Ls) 21.6 cm Length of probe (Lp) 3.56 cm Substrate L1 52.87 mm position L2 58.6 mm Zs Zs = L1 – L2 = -5.73 mm Gas H2 400 sccm (purity grade: 99.9995%) chemistry CH4 12 sccm (purity grade: 99.999%), 3% CH4/H2 151 Table 5.2 – The variable experimental variables in the reactor operating field map measurement. Varied input variables Internal variables Pressure (p) Substrate temperature (Ts) Absorbed power density (Pabs) Microwave power density () As listed in Table 5.1, the experiments for determining the operating field maps for Reactor B employed 2.54 cm diameter and 1.5 mm thick n-type silicon wafers as substrates. Each wafer was pre-seeded by using the scratching method that was described in Chapter 4. The polycrystalline diamond holder used in the experiments was also described in Chapter 4. It was noted earlier [68] that the operating field map also varies with the reactor substrate position geometry variable Zs. Thus here the operating filed map measurement process has been simplified by assuming an experimentally “optimized” substrate position Zs = -5.73 mm. This choice for the Zs position guarantees a stable plasma discharge position in close proximity and contact with the substrate surface [79]. This position was held constant for all operating field map measurements. Thus L1 and L2 were constant. During experiments the reactor was well matched by adjusting Ls, and then after this initial adjustment Ls was also held constant. Usually Lp was not changed once the excitation mode was ensured. 152 The hydrogen and methane input gases had purity grades of 5.5N and 5N respectively, and were the same gases that were used in single crystal diamond (SCD) synthesis presented in this dissertation. A methane concentration of 3% was chosen. Thus the experimental conditions that were used for determining the operating field map were almost the same as the conditions that were used in previous work [1, 86], except for the tuning of the reactor (Ls, Lp) and the adjustment of the substrate position (L1, L2). The tuning of the reactor and the adjustment of the substrate position were especially made for the conditions that were required for the SCD synthesis experiments performed in this thesis work. The results of the operating field map measurement were compared with previous works, and also represent the reactor’s behavior for the SCD synthesis process. 5.2.2 The data acquisition in field map measurement The operational processes for the experiments that were used to determine the reactor operating field map, including the start up and shut down processes, were the same as the polycrystalline diamond deposition processes, that were described in Chapter 4. With the exact reactor tuning and substrate position listed in Table 5.1 above, the excitation of a single TM 013 + TEM001 hybrid mode, the matching of the reactor, and the formation of a discharge on top of the substrate holder were ensured. 153 In the experiments, once the discharge was created, and the pressure was increased to a specific operating pressure condition, the operation of the reactor was then experimentally evaluated by varying the reactor input variables, i.e., the pressure and absorbed microwave power. In the process, the substrate temperature was measured, and the picture of discharge was taken. Then the volume of discharge, Vd, was estimated from the photographs of the discharge and thus the microwave power density could be determined. The details of the data acquisition process are discussed as follows. 5.2.2.1 Substrate temperature measurement For both polycrystalline diamond and SCD synthesis experiments, the substrate temperature, Ts, was monitored by using a portable optical emission pyrometer (IRCON Ultimax Infrared Thermometer UX CL1). The pyrometer measured one fixed wavelength (“one-color”) at 0.96 µm. The temperature range of reading was around 600-3000 ° C. The emissivity used in the measurement of the operating field map was 0.6, the same as that for polycrystalline diamond deposition on silicon. As shown in the MPACVD overall experimental setup system diagram, i.e. Figure 3.11, the pyrometer was placed beside a screened window that was cut through the cavity wall. In order to keep the measurement results consistent, the pyrometer was placed on a tripod (not shown in Figure 3.11). 154 The position and height of the tripod were not changed during all of the field map measurements. Since the substrate position was below the window, the thermal emission was focused through the quartz bell-jar at an incident angle of approximately 60°on the center of the sample. The measurements were usually taken several times for each condition and the most repeatable reading was recorded. Once the pressure or absorbed microwave power was changed, one had to wait a few minutes before a stable substrate temperature reading was obtained. Although the position of the pyrometer, the incident angle, and the measured position on the sample were held constant, it was understood that due to the intense emission from the discharge the measurement of the absolute value of the substrate temperature by a pyrometer might not be accurate. However the measurements presented here were repeatable and thus serve from run to run as a relative experimental substrate temperature measurement. Two-color pyrometer temperature measurements were also taken. Under the same operating conditions the two-color measurements were approximately 100 K higher than the one-color measurements. All the substrate temperature measurements in this dissertation were from one-color measurements. 155 5.2.2.2 Photography of the reactor microwave discharge During the reactor operating field map measurements, a series of photographs were taken of the discharge hovering over the substrate, as the pressure and microwave power varied. As shown in Figure 3.11, the photographs were taken through another window located on the cavity side wall than the one used for substrate temperature measurement. The window was also just above the top of bell jar. A CANON EOS 20D camera was used to take photographs with a 40 mm lens. In order to take photographs with similar background and comparable fixed reference position, the camera was also located at a fixed position on a tripod outside the reactor at an incident angle of around 60° . The camera settings used to take photographs in the reactor operating field map measurement are summarized in Table 5.3. For the purpose of comparison between the photographs, most of the parameters were held constant for all experimental conditions. The recoding mode was set as manual exposure mode. The aperture value and ISO were set to F/4 and 1600 respectively. The position of camera and substrate were both fixed. When the picture was perfectly focused on the substrate rather than the bell jar or cavity window, the focal length was constant at 40 mm. As shown in Table 5.3, the camera exposure setting was kept constant at a given pressure. However since the discharge intensity varied with pressure, the exposure time was adjusted between different pressures so that the photographed discharge 156 appeared similar to that seen by the naked eye. Table 5.3 – Camera settings for discharge photograph. Camera model Canon EOS 20D Recoding mode Manual exposure Focal length 40 mm Aperture value F/4 Exposure time 60 torr 1/100 sec 80 torr 1/200 sec 100 torr 1/320 sec 120 torr 1/500 sec 180 torr 1/2000 sec 240 torr 1/4000 sec ISO 1600 Flash used No Color representation sRGB Exposure compensation No Metering mode Average Figure 5.2 shows a few examples of discharge photographs that were taken at different pressures with different exposure times for a same field map measurement experiment. The pictures in Figure 5.2(a) and (b) were taken 157 with same exposure time of 1/2000 sec, and at pressures of 180 torr and 240 torr respectively. The discharge in (b) has larger volume and was brighter than the discharge in (a). However as a matter of fact, as seen by the naked eye, the discharge at 180 torr with 2.636 kW absorbed microwave power was larger than the discharge at 240 torr with 1.964 kW absorbed power. The glowing substrate in (a) also shows that the discharge in (a) has more power than the one in (b). Thus an adjustment was made in exposure time so that in the photograph the discharge appeared similar to that observed by the naked eye. Since the discharge became more intense as pressure increases, the exposure time should be decreased as shown in Table 5.3. The pictures in Figure 5.3 (c) and (d) were taken at same pressure of 240 torr with same exposure time of 1/4000 sec. As absorbed microwave power increases from 2.466 kW to 2.753 kW, the discharge gets brighter and larger. This observation is consistent with the observations of the discharge by naked eye. However, compared with the picture from (b) at same pressure with longer exposure time, the discharge with lower power looks brighter and larger than the one with higher power. Thus if one would look at just the photographs, they might make conclusions that are different from the observations that are made from the naked eye. Thus for a given pressure, the camera exposure time should be set constant. In conclusion, in order to be able to compare and analyze the photographs scientifically and reliably, the camera parameters were carefully 158 set in the operating field map measurement. At same pressure, all settings were kept the same. As pressure increases, the exposure time was reduced to ensure that the discharge photographs taken were close to what one sees with the naked eye. Figure 5.2 – Examples of discharge photographs: (a) p = 180 torr, Pabs = 2.636 kW, exposure time = 1/2000 sec; (a) p = 240 torr, P abs = 1.964 kW, exposure 159 time = 1/2000 sec; (c) p = 240 torr, Pabs = 2.466 kW, exposure time = 1/4000 sec. (d) p = 240 torr, Pabs = 2.753 kW, exposure time = 1/4000 sec. The other issue in discharge photography was that in the higher pressure regime the photographs taken with short exposure times were not completely repeatable. At high pressures, i.e. > 180 torr, the lack of precision of the photographs would affect the accuracy of microwave power densities that were calculated based on the photographs. Figure 5.3 shows an example of discharge photographs that were taken under the same conditions within intervals of a few seconds. The pressure was 240 torr, the absorbed microwave power was 2.115 kW and the exposure time was set as 1/4000 sec. Within one minute, the three photographs in Figure 5.3 show an appreciable difference in discharge brightness/size with the same experimental condition and camera setting. Thus the microwave power densities determined from those photographs varied. Visually to the naked eye it appeared that the discharge flickered with respect to time. A small amount of variation of the input power versus time was believed as the main reason of this phenomenon. In the MPACVD experimental system, the microwave energy is supplied by a 6 kW Cober (model S6F/4503). The input power was not perfectly continuous and smooth. The power supply power was provided by input 60 Hz power, which then was rectified to 120 Hz. The variation frequency of the discharge was measured using a simple set up 160 that employed a photoresistor to detect the variation of discharge emission. The result of this measurement showed that the discharge flickered with a frequency of 120 Hz, which was related to the unfiltered variation of the input power at the same frequency. As operating pressure increased, the flickering of discharge became more distinct, and occasionally the discharge actually went out. Figure 5.3 – Examples of discharge photographs taken under the same condition: pressure = 240 torr, Pabs = 2.115 kW, exposure time = 1/4000 sec. It is pointed out here that the variation of the input microwave power not only resulted in the discharge flickering, also may influence the diamond synthesis itself. The input power variation may behave similar to a pulsed wave mode with 120 Hz frequency that is close to the 150 Hz pulsed excitation frequency as was described in the work by A.B. Muchnikov et al. [61, 62]. In future research activities it is worth investigating that whether the pulsing like input power helped increase diamond growth rate compared with continuous 161 wave mode input power. However it was not covered in this dissertation research. For discharge photography, the exposure time must be longer than 1/120 sec to avoid the interference of discharge flickering. In order to take useful photographs with long exposure time, filters might be needed to filer part of the emission. That is also a possible direction to explore in future work. In this work, the method of getting better precision was to take several photographs for a same condition and then to take an average. It was worth noting that for the measurements that were done by taking only one photograph for each condition, there was a variation of 10-15% for microwave power densities. For measurements in high pressure regime (> 180 torr), the discharge flickering and the variation of microwave power densities due to the unfiltered variation of the input power became important and obvious. 5.2.2.3 Determination of microwave power density The discharge volume, Vd, was determined from the discharge photographs. Then the discharge average absorbed power density was calculated from = Pabs /Vd. An example of discharge photograph with the calibration is shown in Figure 5.4. The shape of discharge was considered as a perfect sphere. The boundary of the sphere was identified at the border area of the bright core and 162 the light greenish plasma, as shown by the arrow in Figure 5.4. The calibrations in the figure were done by using an image processing computer program (Image Pro plus 5.1). Then using the diameter of silicon wafer (2.54 cm) as reference, the diameter of the plasma ball could be determined as 2.48 cm. Figure 5.4 – Example of discharge photograph with scale for power density calculation. Pressure = 180 torr, Pabs = 2.028 kW, exposure time = 1/2000 sec. 163 With the absorbed microwave power of 2.028 kW and the discharge diameter of 2.48 cm, the microwave power density from this photograph was calculated as follows: 3 3 < Pabs > = (2.028 kW x 1000) / (4/3 x π x (2.48 cm/2) ) = 253.93 W/cm . It was recognized that the method described above to determine the discharge’s volume was not precise, since it depended on photographs taken with different exposure settings and the photographs were sensitive only to visible emission lines and at times from the flickering plasma as discussed above. The photographs did not directly measure the exact volume of ionized plasma region. However, during the time period of this thesis research there were no other better methods to estimate the discharge volume. The photography and calculation processes were controlled and specified to reduce the variation of acquired data as described in this section. At this time the method of determining plasma’s volume by taking calibrated photographs was believed the best. In future research activities, when the experimental results are compared to numerical modeling results, the improved diagnostic methods are expected be developed. 164 5.3 Experimental Reactor operating field map As described above, when experimentally determining the reactor operating field map, the operation of the reactor was experimentally evaluated by varying the reactor input variables, i.e. absorbed microwave power and pressure. One of the internal variables, i.e. the substrate temperature, was measured in the process. Discharge photographs were taken to determine the plasma volume and microwave power densities. These experimental procedures resulted in a set of operating field map curves with discharge photographs. Examples of the operating field map curves are shown in Figures 5.5-5.7, and are described in more detail below. When operating with a given fixed reactor design, the substrate temperature was a function of both the pressure and the absorbed microwave power. Thus given a fixed experimental condition, such as constant input gas mixture and flow rate as described in this chapter, the major independent input experimental variables, i.e. absorbed microwave power, Pabs, and pressure, p, had an experimentally repeatable, nonlinear relationship with the substrate temperature, Ts [69, 89]. When explored over the vast input experimental space this relationship was not only nonlinear but was also characterized by operating discontinuities and hysteresis as the plasma size varies and moves around the discharge chamber. The experimental data presented in Figures 5.5-5.7 and in the paragraphs below only describe the experimental 165 performance when the discharge remained in close contact and was attached to the substrate. Two typical reactor experimental performance curves that display the variation of the substrate temperature Ts, versus absorbed power, Pabs, are shown in Figure 5.5. The lower curve displays the substrate temperature, Ts, versus Pabs behavior for a constant operating pressure of 60 torr which was a typical low operating pressure in CVD diamond synthesis, while the upper curve displays similar reactor operating behavior for a constant operating pressure of 240 torr, which was a typical high operating pressure. For each curve shown in Figure 5.5 the maximum input power, Pabs, was the absorbed microwave power that produced a discharge that was slightly larger than the 2.54 cm diameter silicon wafer substrate. Thus the discharge was able to cover the processing region but was still far from the reactor walls. Next to each curve are a series of photographs of the discharge versus absorbed power. These photographs were taken at a constant pressure using the photography techniques that were discussed above in Section 5.2. The photographs display the discharge’s visual appearance as the input power was varied. The power listed above each photo is the absorbed power, P abs, which maintains the discharge. 166 Figure 5.5 – Examples of the operating field map curves along with the associated discharge photographs for 60 torr and 240 torr for Reactor B. As shown in the photo inserts, the discharge was hovering over a 2.54 cm diameter silicon wafer. Several observations could be made from the data shown in Figure 5.5. Each curve shows that as absorbed power was increased from ~1.1 kW to 1.7-2.7 kW both discharge size and substrate temperature increased. The substrate temperature increased faster with absorbed power at higher pressures than at lower pressures, i.e. the slope ΔTs/ ΔPabs was higher at high pressures than at low pressures. The discharge intensity and color were also different at the two pressures. The discharge was purple at low pressures and 167 it became more intense and white-green at high pressures. At constant absorbed power, for example ~1.7-1.8 kW, the discharge at 240 torr, which was shown in second left picture at the top of Figure 5.5, was much smaller than the discharge at 60 torr, shown in the far right picture at the bottom. Thus the discharge power density was much higher at high pressures than the power density at low pressures. This behavior was consistent with the discharge behavior that has been observed earlier in other high pressure microwave discharges [66-68, 70, 71, 86]. A complete family of experimental curves, where the behavior was measured for many constant pressure conditions, was identified as the “operating field map” for the reactor [67, 69, 89]. These experimental performance curves together with the discharge photographs defined the experimental behavior of the reactor and described the associated discharge appearance as the operating pressure and power were increased. Figure 5.6 displays an example of such a family of curves for Reactor B when it was operated with the specific, fixed experimental conditions summarized in Table 5.1. Also plotted in Figure 5.6 is the average discharge absorbed density versus input power for a number of constant pressure experimental runs. As expected the discharge absorbed power density increased as pressure increased. However, at each constant pressure operating condition the absorbed power density only slightly changed as the input power was increased (see the expanded power density insert in Figure 168 5.6). Under these operating conditions as the input power increased the discharge volume also increased and thus the discharge absorbed power density remained approximately constant or decreases slightly as the input power was increased. As pressure increased from 120 torr to 180 torr, at constant pressure the variation of power density also increased. One factor is that the discharge flickering phenomenon was more distinct at higher pressures than at lower pressures which influenced the precision of discharge photographs and then in turn the calculation results of power densities. On the other hand, at high pressures the plasma volume was reduced. Thus for a smaller plasma volume, a certain variation in discharge sphere diameter measurement resulted in larger variation in the plasma volume calculation, and in turn the larger variation in the discharge power density calculation. If the discharge were to entirely fill the space between the substrate and the quartz walls (this data is not shown in Figure 5.6), the discharge power density would then begin to increase versus any additional increases in input power. This was an undesirable operating condition because if the discharge touched the reactor walls the diamond synthesis process itself was altered due to wall reactions, and as the reactor walls heat up they became an additional thin film deposition surface. At these higher input power levels the quartz walls could even erode thereby contaminating the synthesis process. Additionally, depending on the level of the input power, the discharge might even attach itself to the quartz walls or to surfaces below the substrate in the coaxial 169 section of the applicator. These were identified as undesirable discharge conditions and were a function of pressure, substrate position and input power. Thus determining the reactor operating field map defined an operating region where these undesirable discharge conditions were avoided. Figure 5.6 – Operating field map and the corresponding power density curves for Reactor B. Compared with the operating field map of Reactor B from previous work 170 [1] which only consisted of limited data from the safe and efficient operating region, Figure 5.7 displays an expanded and more detailed version of the Reactor B operating field map. The reactor operation for the measurement was pushed into extreme conditions with minimum and maximum input power. The minimum input power just ensured the plasma remaining on top of the substrate. Exciting the discharge with a lower power than the minimum value, the plasma may no longer exist. The maximum value was defined as the input power when the discharge almost fills the whole space in the bell jar but just before it touches the quartz walls. Within all the data in Figure 5.7, superimposed on the operating field map is an enclosed operating region defined by the green dashed-dot lines. The left green dashed-dot line represents the minimum power required to form a 2.54 cm diameter discharge over a 2.54 cm diameter silicon substrate. Input powers of less than indicated by this line will not produce uniform deposition over 2.54 cm diameters. The right green dashed-dot line represents the input power at which the plasma expanded beyond 2.54 cm diameter. If power was increased further beyond this limit the reactor then is operating in an inefficient condition. If the input power was increased further the discharge might touch the quartz dome. Large input powers resulted in inefficient operation and would cause dome heating and might possibly cause other undesirable discharge and dome wall interactions. Thus the operating space enclosed between the left and right dashed-dot lines represents the allowable operating 171 input power levels that yield useful diamond deposition over a 2.54 cm silicon substrate. The experimental data presented in the enclosed space in Figure 5.7 defines the operating conditions that enable discharge stable, and wall reaction free discharge operating conditions. Operation where the discharge was stably attached to the substrate and also where the wall reactions were minimized was identified here as the efficient, safe and robust method of operation. All the undesirable experimental situations described above were avoided when operating the reactor within the safe and efficient operating region. Since the reactor was also operated in a matched condition during the field map measurements, once the safe, well matched, and efficient operating region was established, diamond synthesis could be performed reliably, robustly and safely versus time within this region. All of the experiments reported for SCD synthesis in this dissertation were performed within this experimental multivariable input volume approximately along and around the thick green dashed line shown in Figure 5.7. It is also useful to note that the operating field map curves shown in Figures 5.5-5.7 were obtained under a fixed experimental condition as described in section above. In the SCD synthesis experiments, the other input variables in the multivariable space could be changed to vary the substrate 172 temperature at a constant pressure. For example, the change in substrate holder configuration could shift the operating field map curves either up or down versus Ts. However the safe and efficient operating regions remain approximately the same as indicated in Figure 5.7. Figure 5.7 – Operating field map curves and the identification of the efficient and safe experimental diamond synthesis region for Reactor B. 173 5.4 High pressure discharge behavior Once the safe and efficient operating region for the reactor was defined, the CVD diamond synthesis could be performed in this region reliably, robustly and safely. Shown in Figure 5.8 are the photographs of discharge at different pressures varying from 60 torr to 240 torr. In these photographs, the absorbed powers that are indicated in the figure were set around the thick green dashed line shown in Figure 5.7. The experimental condition was the same as described in the section above. Figure 5.8 – Photographs of the discharge over the silicon substrate as the pressure was increased from 60 torr to 240 torr. The microwave absorbed power ranged from 1.331 kW to 2.087 kW as indicated. 174 As shown in Figure 5.8, the plasma discharge initially filled the discharge chamber at low pressure of 60 torr. As the pressure was gradually increased the plasma size began to shrink and pull away from the quartz walls at about 60 torr to 120 torr, and gradually became smaller as the pressure reached 240 torr. The discharge color turned from purple at pressure of 60 torr, to violet at pressure from 80 torr to 120 torr, and then to greenish when pressure was greater than 180 torr. The intense white core of the plasma sphere became brighter as pressure increased. At low pressures the microwave discharge filled the space within the bell jar and produced a diffusion loss dominated, cold (gas temperatures < 1000 K), non-equilibrium plasma [1]. While in the high pressure regime the microwave discharge was hot (gas temperature > 3000 K), was volume recombination dominated and became a more thermal-like discharge. As the photographs in Figure 5.8 shown, at high pressures greater than 100 torr, the discharge tended to separate from the quartz walls and became constricted and thermally inhomogeneous. The discharge could become freely floating, even move about the chamber space as it reacted to the buoyant forces on the discharge and to the convective forces by the gas flows in the discharge chamber. Figure 5.8 shows that the discharge becomes more visually intense, and shrinks in volume as the pressure is increased. As a result, the absorbed 175 power density increased. Figure 5.9 shows the experimentally measured absorbed microwave power densities versus pressures for different reactors. Reactor A was the conventional reactor design that was introduced in Chapter 3. Reactor C was another modified reactor design which had the same cooling stage design as Reactor B [86, 90]. Figure 5.9 – Power density versus pressure for Reactors A, B and C. The data of Reactors A and C are from Ref [90]. 176 The green data points of Reactor B in Figure 5.9 were acquired by using the techniques described in this chapter under the fixed experimental condition. 3 The discharge power density increased from about 80 W/cm to about 650 3 W/cm as the pressure increased from 60 torr to 280 torr. Compared with other two reactors, as expected the power densities of the improved reactor designs, Reactors B and C, were much larger than the power densities of the conventional reactor design, Reactor A. The reduction of the center conductor area by about 4.5 resulted in the increasing of the power density by a factor 4-5. Reactors B and C had similar performance as pressure increased. The power densities of Reactor C were larger than the power densities of Reactor B by approximately 10-15%. In conclusion, by operating the MPACVD reactor under controlled experimental conditions, the reactor’s performance at high pressures was evaluated. The operating field map for the reactor was experimentally determined and the safe and efficient experimental operational region was defined for SCD synthesis experiments. On the other hand, the high absorbed power densities that were obtained at high pressures were expected to lead to an increase in the densities of CH3 radical and atomic hydrogen. Thus operation in high pressure regime was also expected to help increase diamond growth rate and improve diamond quality. 177 CHAPTER 6 PROCESS CONTROL AND MATCHING METHODS FOR SINGLE CRYSTAL DIAMOND SYNTHESIS 6.1 Introduction This chapter presents the microwave plasma process control methods that enable the microwave plasma assisted chemical vapor deposition (MPACVD) synthesis of single crystal diamond (SCD). These process control methods make use of the microwave discharge’s intrinsic nonlinear behavior, which is in part described by the reactor’s operating field map that was identified in Chapter 5. The reactor’s experimental operating field map can be adapted and varied to enhance the ability to control the diamond synthesis process. Here in this chapter the process control methods are described that were employed to synthesize SCD. Specific details of the process control methods, such as substrate temperature control versus time, process control via substrate position variation, and reactor matching via Ls adjustment, etc. are presented. Many of the MPACVD diamond synthesis process control methods that are described below can be applied to any microwave plasma reactor design. However the discussions which are presented in this chapter focus on the reactor behavior while operating within the high pressure 178 deposition regime. In particular this chapter focuses on: (1) the internal reactor matching; (2) process optimization methods for Reactor B to achieve efficient microwave coupling and high growth rates over a range of input powers and pressures; and then (3) methods of process control versus time that were employed during the SCD synthesis process cycle. 179 6.2 MPACVD reactor operational principles The goal of controlling and optimizing a microwave plasma process is complex, since the discharge phenomenon are nonlinear and often have multiple, stable operating regimes. The size, shape and position of the discharge and the discharge electron, ion and radical species densities depend on the input power and pressure. The microwave plasma loaded reactor input impedance also depends in a complex way on the input power and pressure. The coupling of microwave energy into a microwave discharge load is a nonlinear matching and control problem and may have a number of stable operating conditions. Thus the appropriate reactor and experimental conditions must be identified and the reactor must be adjusted so that the process takes place within a safe, efficient, high growth rate and high quality diamond synthesis operating regime. The maintenance and control of the microwave discharge becomes increasingly difficult when operating pressure is increased. This phenomenon is associated with the fundamental behavior of microwave discharge as the pressure and the absorbed density were increased. As described in Chapter 5, when pressure is increased beyond 100 torr, the microwave discharge constricts, separates from the quartz walls, and becomes more intense. The discharge at times may become freely floating and assume shapes that are related to the shape of the standing wave impressed electromagnetic fields [70, 180 74]. At very high pressures microwave discharges becomes very non-uniform, intense and “arc-like”. They may become unstable and even move about the discharge chamber. This behavior adds to the complexity of controlling and optimizing the MPACVD synthesis process at high pressures. As the position, size and shape of discharge varied the microwave system could be established in a number of different steady states. However, only one or a few of the steady states are useful for the SCD synthesis process. In addition to the discharge matching, the discharge behavior must be controlled by developing process methods that are experimentally controlled versus time. Since it is also desirable to eliminate or at least minimize reactor wall heating and plasma-wall interactions and to maximize substrate growth reactions, it was necessary to control the position of the hot discharge so that it is not in direct contact with the quartz walls. Additionally in order to improve the diamond synthesis process it was also important to control and position the discharge so that it was in direct contact with the cooled substrate. That is it was also necessary to insure that the high density discharge/substrate boundary layer, i.e. the important discharge region for diamond synthesis [10, 91, 92], was always next to and in good contact with the substrate. Thus while operating in the high pressure regime locating the discharge away from the walls and in good contact with the substrate holder became an additional 181 principle that was established to enable reactor operation and process optimization at high pressures. 182 6.3 Discharge formation, position control and reactor matching strategy As is presented in Chapter 4, there is a multivariable experimental variable space within which the MPACVD experimental reactor must operate to synthesize diamond. A subgroup of these experimental variables is the controllable input reactor geometry variables which consist of variables that are concerned with reactor design and geometry, and substrate holder design, etc. Although the dimensions of the reactor were fixed, the optimized reactor design for Reactor B incorporated several mechanical adjustments that enabled the efficient microwave coupling, and maintenance and control of the size and position of the microwave discharge at high pressures. Specifically there were four mechanical tuning variables, i.e. a group of controllable input variables, Ls, Lp, L1 and L2 (see Figure 3.12), which allow a wide range of operating possibilities. The input variables that are related to the reactor geometry that were used in the reactor internal microwave matching and the discharge position controlling process were Ls, and Lp and substrate position, Zs (i.e. L1 and L2). The availability of these reactor mechanical tuning variables together with the other independently varied input variables provided a multitude of potential operating conditions. An important question was what is the best Ls, Lp, L1 and L2 adjustment strategy that first enables the ignition, maintains the discharge, and then optimizes the process. 183 Figure 6.1 shows a block diagram of the experimental variables used in the plasma controlling and microwave matching processes. This diagram is evolved from Figure 4.9. As shown in Figure 6.1, the block of output variables is shown in grey, which indicates that these variables were ignored during this phase of the development of plasma process control. Among all controllable input variables and independently varied input variables, reactor mechanical tuning variables, Zs, Ls, and Lp, pressure, p, and microwave powers, Pinc, Pref, and Pabs, were the most important input variables identified that allowed the control of and the matching of the discharge. They are listed in the block indicated by the arrow underneath the block of input variables shown in Figure 6.1. In addition to the discharge position, size and shape that could be observed directly, in a similar fashion to the reactor operating field map measurements the internal variables, i.e. substrate temperature, Ts, and power density, , were identified as variables that controlled the reactor’s performance. They are enclosed in the block below internal variables in Figure 6.1. 184 Figure 6.1 – Block diagram of experimental variables in microwave plasma controlling process. As shown in Figure 6.1, a reactor adjustment strategy was imposed on the variation of the input variables, i.e. input power, pressure, substrate position, and reactor geometry. The reactor operational strategy that was adopted is described as follows:  The main functions of Lp and Ls were first to select, match and excite a desired single electromagnetic mode, i.e. in Reactor B the hybrid TM013 (in the cylindrical section) + TEM001 (in the coaxial section) electromagnetic mode was excited in the cavity applicator by the proper adjustment of Ls and Lp. Example discharge ignition positions were Ls= 21.6 cm and Lp = 3.56 cm. Then as the discharge was ignited and formed, and as the process conditions were varied, Lp and 185 Ls were further varied to adjust the reactor to a desired optimum microwave matched operating condition.  The role of L1 and L2 were to position the substrate surface, Z s, in contact with the discharge in order to adjust the discharge boundary layer for optimal diamond synthesis. If Zs was varied then Lp and Ls also might have to be slightly varied to achieve an excellent microwave matched condition and hence insure a high microwave coupling efficiency. The primary role of Ls and Lp was to maintain the desired single mode and then to match the reactor around a stable and desirable operating condition to ensure high coupling efficiency. It is noted here that while operating in the high pressure regime once the discharge loaded hybrid mode excitation was achieved the probe position, Lp, was held constant.  L1 and L2 were adjusted to locate the discharge in contact with the substrate, and to achieve discharge stability (i.e. find a stable Zs discharge operating regime). Then when the reactor was operated within the stable discharge operating space L1 and L2 were further slightly adjusted together with a slight adjustment of Ls to achieve optimal and efficient process conditions on the substrate.  As shown in Figure 3.13, L1, the distance between the top of substrate holder and the bottom of the coaxial section, was only adjusted with 186 variation no more than 2 mm, because the cooling stage design used in this dissertation research was fixed and the substrate holder designs were limited. L2, the length of the coaxial section (z < 0), was able to be varied within in the range of 0-10 mm. This variation was achieved in discreet steps by adding circular stainless shims of various thicknesses (in Figure 3.13). Thus adjustment of Zs was mainly determined by varying the length of L2.  It is useful to note that once the initial length adjustments of the tuning variables were established for electromagnetic mode selection and for the location of the substrate position, then the tuning adjustments required for matching or for the variation of the substrate position, were usually very small and were on the order of a few mm.  In this dissertation research the appropriate substrate position was chosen by fixing L2 at 58.6 mm for most of the experiments reported here. Substrate position, Zs, was then adjusted by varying the substrate holder design. The length of probe, Lp, was kept constant at 3.56 cm. The length of the short, Ls was adjusted as L2 varied. Thus it was positioned at approximately 21.6 cm for most of the experiments presented in this dissertation. Reactor B embodied the principles that were described above. The reactor control processes that are discussed below make use of the reactor 187 tuning variables shown in Figure 6.1 to achieve a stable discharge, and to enable safe, efficient and robust reactor operation for CVD diamond synthesis. The independently varied input variables together with other input variables in Figure 6.1 were also used in the reactor control processes described below to optimize and enhance the diamond synthesis process, such as maintaining an optimal substrate temperature versus time. By employing a number of small adjustments to the input variables, such as input power and pressure, combined with the variation of L1, L2, Ls, and Lp, a large variety of efficient diamond synthesis process cycles could be achieved. 188 6.4 Reactor matching: tuning via variation of Ls As described in the section above, the reactor tuning variables, Ls and Lp were used to select and excite the single hybrid electromagnetic mode, while L1 and L2 were varied to position the discharge in direct contact with the cooled substrate rather than allowing it to move about in the discharge chamber. Once the initial tuning process was established, only very small mechanical adjustments of the variables were needed for matching and discharge control. In particular if either Zs, input power or pressure, p, are varied then just a small adjustment of Ls enabled a very well matched and hence high microwave coupling efficiency condition to be established. In the diamond synthesis experiments, substrate position, i.e. L1, L2 and Zs, were not able to be changed during a specific process run, since all the substrate position adjustments for Reactor B were made inside the discharge chamber. However Ls and Lp could be adjusted from outside the discharge chamber during the process. As a matter of fact as described in Section 6.3 above, Reactor B was already well matched [1], thus Ls was only slightly adjusted when L2 was varied as different experimental runs were performed. Figure 6.2 displays an example of the sensitivity of the variation of Ls to the reactor matching versus absorbed power when all other experimental variable are held constant. In Figure 6.2 there is one reactor operating field map curve of substrate temperature versus absorbed power for a constant Ls 189 of 21.6 cm. This curve lies on the top of the figure and is displayed with the solid red curve and symbols. The three curves with hollow symbols in the bottom display the variation of Pref/Pinc versus absorbed power as Ls is varied about 21.6 cm. The experimental conditions for data acquisition were the same as the conditions that were described for the reactor operational field map measurements in Chapter 5. The experiment was performed by using a 2.54 cm diameter silicon wafer as substrate. The operating pressure and methane concentration were 180 torr and 3%, respectively. The three other tuning variables were held fixed: Lp was constant at 3.56 cm, L1 and L2 were also fixed at 52.87 mm and 58.6 mm respectively. The substrate position Zs was held fixed at -5.73 mm. As discussed in Chapter 3, when the microwave power absorbed by the conducting walls and the by other dielectric materials, i.e. Ploss, was small (< 2%) and therefore could be neglected, the microwave coupling efficiency to the discharge, Effcoup was given by the following equation [65, 70-73], Effcoup = 1 - Pref/Pinc x 100%. Thus the minimum of Pref/Pinc in the curve yields the maximum of microwave coupling efficiency. 190 Figure 6.2 – Variation of the Pref/Pinc versus absorbed power versus different Ls positions. The curve on the top is the field map curve for substrate temperature (left axis) versus absorbed power when Ls is held fixed at 21.6 cm. Experimental conditions: 2.54 cm diameter silicon substrate, p = 180 torr, CH4/H2 = 3%, Lp = 3.56 cm, L1 = 52.87 mm, L2 = 58.6 mm, Zs = -5.73 mm. The curves of Pref/Pinc versus absorbed power which are shown in Figure 6.2 display the sensitivity of Pref/Pinc when Ls was slightly varied. As shown in Figure 6.2, generally in the optimally designed and operated Reactor B, under the experimental condition described above, the microwave coupling efficiency was very high. If all experimental conditions are held constant and the absorbed power and Ls are varied then the value of Pref/Pinc varies. When 191 Ls is 21.6 cm, the Pref/Pinc value reaches minimum in the wide range of absorbed power of 1.4-2.7 kW, where all Pref/Pinc values are all less than 5%. This means that the operation with Ls of 21.6 cm was always under well matched condition. In the absorbed power range of 1.9-2.3 kW, the Pref/Pinc was obtained around 1%, which was able to ensure a very high microwave coupling efficiency of almost 100%. The operating field map curve with red solid symbol was measured with Ls of 21.6 cm. As shown in the other two curves of Pref/Pinc versus absorbed power in Figure 6.2, when Ls, the length of short, was increased or decreased by 1 mm, the microwave coupling efficiency dropped to less than 97%. When the absorbed power was set in the safe and efficient operating region around 1.9 kW, the microwave coupling efficiency was obtained around 95%, which was still able to be recognized as a well matched condition. However, if the input power was adjusted out of the regime, the Pref/Pinc might increase fast and the microwave coupling efficiency would deteriorate quickly, and reflected power would increase, which might increase the risk of metallic reactor wall heating. However, additional iterative small adjustments of Ls and Lp (not shown in the figure) can reduce the reflected power and bring the reactor back into a well matched condition. This shows that the Pref/Pinc value was very sensitive to the variation of Ls. During SCD synthesis experiment a high microwave coupling efficiency could be achieved by varying Ls. 192 Additionally, when the absorbed power was pushed to the extreme of more than 2.5 kW with Ls of 21.5 cm in the measurement, a 1 cm diameter area of hot spot was observed on the quartz wall of bell jar dome, which proved the importance of operation under safe, efficient and well-matched operating conditions. It is worth noting that in the Reactor B operating field map (see Figure 5.7) the safe and efficient operating region around the thick green dashed line was also in the high microwave coupling efficiency range. Take 180 torr as an example, the absorbed power for safe and efficient operation indicated in Figure 5.7 was around 1.7-1.9 kW, where the microwave coupling efficiency of 98-99% was obtained. In the curve of 180 torr in Figure 6.2, this absorbed power range of 1.7-1.9 kW is in the lower power end of high microwave coupling efficiency range. In order to obtain high efficiency and also to reduce the possibility of wall heating or wall-plasma interactions during SCD synthesis experiment, the experiments performed for this dissertation research tended to be operated with input power levels in the lower range. Figure 6.3 shows the curves of Pref/Pinc versus absorbed power at pressures of 120 torr, 180 torr and 240 torr when Ls was held fixed at 21.6 cm. The other experimental parameters were held at the same as the conditions that are shown in Figure 6.2, and the reactor was well matched. It also shows the reactor operating field map curves at these pressures. 193 Figure 6.3 – Substrate temperature and Pref/Pinc versus absorbed power at pressures 120 torr, 180 torr and 240 torr when the reactor was operated in a well matched. Experimental conditions: 2.54 cm diameter silicon substrate, CH4/H2 = 3%, Lp = 3.56 cm, Ls = 21.6 cm, L1 = 52.87 mm, L2 = 58.6 mm, Zs = -5.73 mm. Similar to the coupling efficiency curve in Figure 6.2, there was a minimum of Pref/Pinc where the reactor could reach the highest microwave coupling efficiency. For each operating pressure, the absorbed power where the highest coupling efficiency could be obtained was different. It was around 1.3 kW at 120 torr, increased to 1.9 kW at 180 torr, and reached 2.5 kW at 240 torr. In actual SCD synthesis experiments reported in this dissertation, the 194 absorbed power applied was not exactly at the peak of these curves, but was always in the high microwave coupling efficiency range of >95%. Taking the operational pressure of 240 torr as an example, the absorbed power was usually set in the range of 1.7-2.0 kW, in order to get high coupling efficiency and to operate the reactor safely and robustly as well. This operating range could also be seen in the safe and efficient operating region around the thick green dashed line, as is shown in the Reactor B operating field map in Figure 5.7. 195 6.5 Substrate temperature control versus time during the SCD synthesis process Once the safe and efficient process window was determined then in order to achieve an optimal synthesis process the operation of the reactor was controlled versus time by appropriately varying the input variables within this process window. In response to the input variable changes the internal process variables such as substrate temperature, discharge power density, and discharge size, shape and position could be varied. During a MPACVD SCD synthesis experimental run, substrate temperature is a very important variable. Being one of the internal variables in the multi-variable space, substrate temperature depends on the input variables, including the tunable reactor geometrical variables and independently varied process variables, and thus was controllable. Compared with other internal variables, the substrate temperature was easier to monitor during an experiment than the plasma volume or the power density. The substrate temperature measurement was regarded as a reference measurement that enabled one to understand the repeatability of each individual experiment. Since the diamond growth rate and quality cannot be observed and measured during the experiment, it was useful to relate the experimentally measured substrate temperature to diamond growth rate and diamond quality. This is discussed in details in Chapter 7. 196 This section presents the methods of substrate temperature control versus time that can be employed during SCD synthesis experiments. 6.5.1 Determination of SCD substrate temperature As described in Chapter 5, the substrate temperature in SCD synthesis experiments, Ts, was monitored by using a portable optical emission pyrometer (IRCON Ultimax Infrared Thermometer UX CL1). The pyrometer measured one fixed wavelength (“one-color”) at 0.96 µm. The temperature range of reading was around 600-3000 ° The emissivity used in the SCD C. synthesis experiment was 0.1, for diamond deposition on a single crystal diamond substrate. Using the same method of temperature measurement as described in Section 5.2.2 for determining the reactor operating field map, the pyrometer was placed on a tripod with fixed position beside a screened window on the cavity wall. The pyrometer was focused through the window and the quartz bell-jar at an incident angle of 60°on the substrate surface. Since the SCD substrate surface was only 3.5 mm x 3.5 mm, the substrate temperature was identified as the temperature obtained when focusing on the center of substrate. Otherwise the reading might be interfered with the temperature of substrate holder. Since the pyrometer vibrated slightly due to the blowing of the surrounding cooling fans, the temperature reading varied with a variation of 197 approximately 10 ° in a measurement. Thus the measurement was usually C taken several times and the most repeatable reading was recorded. In the reactor operational field map measurement, the substrate temperature was measured as experimental conditions were changed. However in the SCD deposition experiments, the experimental conditions were held constant once the reactor was well matched and the process cycle was initiated. During the deposition process cycle the substrate temperature was measured versus the operation running time. This was usually every few hours. The final substrate temperature of a SCD deposition temperature was identified as the average value of the temperature readings during the whole experiment. The process of determining the substrate temperature is described in Figure 6.4. This figure shows an example of a series of substrate temperature measurements in a SCD deposition experiment over 8 hours. The scattered experimental temperature measurements are linear fit as the red line shows in the figure. Then the average substrate temperature is determined as the median value of the linear fit, i.e. the value in the fit line when running time was 4 hours in the 8-hour experiment. In this example, the linear regression of all measurements is Y = 1055.71 + 0.73 * X. When X is 4, the average substrate temperature was calculated as 1059 ° C. 198 This method of measuring the SCD substrate temperature was used in all SCD deposition and etching experiments reported in this dissertation. All the recorded SCD substrate temperatures were the average value over the entire process cycle. Figure 6.4 – An example of linear fit of substrate temperature measurements in an 8 hour SCD synthesis experiment. 199 6.5.2 Controlling the substrate temperature versus time The reactor operating field map was determined under a certain experimental condition with fixed process input variables. When the input variables, such as reactor tuning variables Ls and Lp, and substrate position variables L1 and L2, are adjusted the shape of the operating field map curves also are changed. The operating field map curves can also be modified by varying the reactor design or by varying the substrate thermal management. During the synthesis process diamond might grow on the substrate surface and also on the molybdenum substrate holder. Thus the reactor behavior and its operating field map curves vary slightly versus deposition time as the substrate local thermal conditions vary due to the deposition of diamond on the substrate. This is shown in Figure 6.5 where the initial process temperature increases from the operating point A (deposition time t = t 0 = 0) to point B (deposition time t = t1 > 0) for a same input absorbed power, as the operating field map curves shift from curve 1 to curve 2 during the process cycle. As the diamond grows, the thickness of diamond on the substrate increases, polycrystalline diamond is also deposited on the surface of the molybdenum substrate holder and the diamond substrate surface temperature increased from Ts to Ts’, where Ts’ –Ts < 100 ° with constant absorbed C, power. As shown in Figure 6.5, the operating field map curves varied slightly 200 as time increased from t0 to t1 and the reactor operating point varied from A to B. Figure 6.5 – Variation of the operating field map, i.e. from curve 1 to curve 2, versus synthesis time, t. Under many synthesis process cycles it was desirable to adjust the reactor versus time to compensate for these physical and associated substrate temperature versus time variations, and to hold the substrate temperature constant versus time. Holding the substrate temperature constant over the process cycle was also desirable in the experimental investigation of understanding the relationships between substrate temperature and other 201 variables. Although the substrate temperature in the research was identified as the average value during the whole deposition process, it was expected to maintain the substrate temperature within a range as small as possible when considering it as a process variable. The method of achieving this is discussed in more detail below. As can be observed from the operating field map curves of Figure 5.7, at higher pressures the slope of the operating field map curves, i.e. ∆Ts/∆Pabs, is larger than the slope at lower pressure. Thus for the SCD synthesis experiments that are performed at pressures greater than 180 torr in this dissertation, the large slopes of the operating field map curves suggested that when operating within the desirable range of substrate temperature at a constant pressure the substrate temperature could be precisely controlled over the multi-hour synthesis process by making small adjustments in input microwave power. As shown in Figures 6.5 and 6.6, the operating field map curves varied from curve 1 to curve 2 after deposition of a period of time. However as also shown in Figure 6.6, the substrate temperature could be held approximately constant by slightly adjusting the input power from Pabs to Pabs’. For example by reducing the input power by just 50-100 Watts the substrate temperature could be held constant at Ts (see operating points C in Figure 6.6). 202 Figure 6.6 – Controlling the input power versus time to achieve constant substrate temperature versus time. The temperature was held constant at the initial temperature by moving the reactor operating point from A to C as the input power was reduced. The absorbed power can be adjusted in either of two different ways. One way was to directly vary the incident power by adjusting the microwave generator output power, which is indicated in Figure 6.6. A second method of absorbed power variation was by holding incident power constant and slightly varying Ls to detune the reactor from a match and thereby mismatching the reactor and slightly increasing the reflected power, Pref. Thus the absorbed power could be adjusted. However in the SCD synthesis experiments reported 203 in this dissertation, in order to create an experimental condition as constant as possible, the Ls was not adjusted once the reactor was well matched. In this dissertation only the first method of absorbed power variation was used in all the experiments to achieve constant substrate temperature versus time. Figure 6.7 shows an example of substrate temperature adjustment by varying absorbed power during an SCD deposition experimental run. The data used in the figure were the same as used in the example of average substrate temperature calculation that is shown in Figure 6.4. In this eight-hour SCD deposition experiment, the goal of substrate temperature controlling was to keep it as close as possible to 1060 ° C. At the beginning of the experiment, the substrate temperature was below 1040 ° C. Then it went up as the deposition time increased and got close to 1060 ° as is shown in Figure 6.7. C When the substrate temperature was higher than the target temperature of 1060 ° the absorbed power was decreased by 50-100 Watts in order to C, adjust the substrate temperature down by 15-20 ° as shown in the most left C, circled data points in Figure 6.7. The circled data in this figure shows that a similar adjustment of substrate temperature occurred three times over the process cycle. According to the calculation described above, the final average substrate temperature of this experiment from these measurements was 1059 ° C. Thus the purpose of adjusting the substrate temperature around 1060 ° was achieved. C 204 Figure 6.7 – Example of substrate temperature adjustment by absorbed power variation during SCD deposition experiment. The circled data represents the substrate temperature variation when the absorbed power was changed. The substrate temperature can also be controlled by holding the absorbed power constant, and then varying the operating pressure. Figure 6.8 displays this method of process control. As shown the reactor operating field map curve varied from curve 1 to curve 2 versus time. By reducing the operating pressure from p1 to p2 (p2 < p1), the reactor operating field map curve changes from curve 2 to curve 3, and then the substrate temperature versus time can be held constant. 205 Figure 6.8 – SCD synthesis process control via varying the pressure from p1 to p2. Curve 3 represents the operating field map curve at t =t1 and a pressure of p 2. It is worth noting that as the pressure was varied the discharge size, position and power density would vary slightly. This method might not be appropriate to be used when all the experiment conditions are expected to be kept as constant as possible. 206 6.5.3 Controlling the substrate temperature via variation of substrate holder design When other experimental variables, including the reactor geometry variables, and experimental condition variables, such as pressure, absorbed power, etc. are kept constant, the thermal environment in the small region between the substrate cooling stage and the discharge, i.e. the substrate holder region, has the major influence on the substrate temperature. As described in Chapter 3, the substrate holder for SCD synthesis was designed as a “pocket” holder. There is a “pocket” in the substrate holder in which the SCD substrate is placed. The substrate holder thickness, the depth and the thickness of the pocket are all important factors in determining the local thermal conditions of the substrate. Once other variables were fixed, the shape and position of discharge only changed a bit as the substrate holder varied. The depth of the pocket determines the distance between the discharge and the substrate top surface. The closer the substrate is to the discharge, the higher the substrate temperature. On the other hand, the substrate temperature is also influenced by the distance between the substrate bottom and cooling stage. The shorter distance insures better substrate cooling conditions, and the lower substrate temperature. Thus the substrate temperature can be controlled by varying the substrate holder design. 207 As shown in Figure 3.17-3.20, among all substrate holder designs, there was only one “one-piece” substrate holder for SCD synthesis. Thus its design was fixed. The design of two-piece substrate holder was changed by varying the design of the substrate holder top piece. This dissertation research used a series of substrate holder top pieces with various depths and thicknesses. The details of the configurations of all substrate holders used in this research are summarized in Appendix A. The example of substrate temperature variation by using different substrate holder is described in more detail with the deposition results in Section 7.4.1. 208 6.6 Process control via substrate position variation The ability to perform external tuning provided the MPACVD reactor design considerable process control. The multiple adjustments of Ls, Lp, L1 and L2 allowed one to simultaneously match the reactor and still control the size, shape, position and power density of the discharge. Additionally while operating at a constant pressure the substrate temperature and the discharge power density could also be varied. For example, if the reactor was operating at 240 torr within the safe and efficient operating regime as shown in Figure 5.7, the substrate position, Zs, could be varied from +1 mm to -10 mm. Then the electric field, the discharge shape and position could be varied in the space above and around the substrate as Zs was varied. The variation of electric field versus Zs was investigated for Reactor C [90]. The simulation results showed that as the substrate position varied from + mm to – mm the electric field above the substrate became more focused and more intense. When the discharge was present it was experimentally observed that the discharge behaved in a similar fashion. This behavior was also true for Reactor B. More specifically when a discharge was operating at a constant pressure the discharge power density and substrate temperature also varied as Zs was varied. As the substrate position was varied from + mm to – mm, similar to the electric field, the discharge became smaller, more non-uniform 209 and intense, and more focused onto a smaller region in the center of the substrate. The study of the variation of absorbed power density and substrate temperature versus Zs for Reactor B was done in a previous work [1, 68] when investigating polycrystalline diamond deposition. The results are shown in Figure 6.9. This figure shows that both the discharge power density and the substrate temperature increase as Zs moved from a positive to a negative substrate position. Though the uniformity of discharge becomes worse as Zs was reduced, for SCD synthesis where high rate diamond synthesis over just a 2 few cm was desired, the process operation at negative z position was more appropriate. Thus the SCD process can be optimized by adjusting the substrate position either in coordination with other process variables or by only varying Zs while holding all the other variables constant. The variation of substrate temperature by adjusting substrate temperature for SCD synthesis using Reactor B is described with the SCD deposition results in Chapter 7. 210 Figure 6.9 – Variation of absorbed power density and substrate temperature versus substrate position for Reactor B by polycrystalline diamond deposition [1]. Experimental conditions: p = 240 torr, CH4/H2 = 3%, L1 = 5.65 cm. 211 CHAPTER 7 MICROWAVE PLASMA ASSISTED CVD SINGLE CRYSTAL DIAMOND SYNTHESIS: EXPERIMENTAL RESULTS 7.1 Introduction This chapter presents the experimental results in the high pressure regime of 180-280 torr for single crystal diamond (SCD) synthesis by using the microwave plasma assisted chemical vapor deposition (MPACVD) Reactor B. The MPACVD Reactor B and the associated experimental system are described in Chapter 3. The procedures for the reactor operation and SCD synthesis are described in Chapter 4. These experiments were all operated within the safe and efficient operating regime which is defined in Chapter 5. A brief investigation of the plasma etching pre-treatment of SCD seed substrates is also presented in this chapter. The experimental results of SCD synthesis is discussed within the multi-variable experimental variable space. In particular, the relationship of input variables, such as substrate temperature, pressure, and methane concentration, and output variables, such as growth rates, and quality is investigated. Thus a high quality, high growth rate SCD synthesis window is experimentally identified. Furthermore, initial exploratory experiments concerned with the synthesis of large area SCD plate production 212 are described. 213 7.2 The experimental variable space for single crystal diamond synthesis As described in Chapter 4, the potential experimental variable space for MPACVD diamond synthesis is vast. In the experiments for determining the reactor field map, only a few important diamond deposition conditions were chosen, and most of input variables, such as reactor tuning variables and input gas chemistry, were held constant. However, since one of the major objectives of this dissertation research was to explore the vast multi-dimensional experimental process variable space and to identify a process recipe for high growth rate, high quality SCD deposition, a few additional input variables were included in the SCD synthesis experiments. Figure 7.1 displays a block diagram of the experimental variables that were varied and investigated in the SCD synthesis experiments that are reported in this chapter. This diagram is also evolved from Figure 4.9. As shown in Figure 7.1, the specific variables that were varied and investigated in the SCD synthesis experiments are listed below the blocks in the groups as indicated by arrows. The internal variables, substrate temperature, T s, and microwave power density, , are enclosed below the block of “internal variables” as shown in Figure 7.1. Substrate temperature was regarded as an experimental reference to the repeatability of SCD synthesis experiments. It was expected from what was already known about the MPACVD synthesis of 214 polycrystalline diamond films at lower pressures [9], that one could vary the SCD synthesis rates and quality by varying the substrate temperature. As discussed in Chapter 6, substrate temperature could be controlled by many methods, such as variation of absorbed power, pressure, substrate position or substrate holder design. Figure 7.1 – Block diagram of experimental variables in SCD synthesis experiments. The specific controllable and independently varied input variables are also shown in Figure 7.1. Among them, input gas chemistry was another important input variable that was expected to influence the diamond deposition results. When the length of the coaxial section of the reactor (in Figure 3.12), 215 L2, was varied by adding circular stainless shims of various thicknesses (in Figure 3.13), the length of short, Ls, was also readjusted in order to achieve a matched reactor as is described in Section 6.3. The output variables, diamond growth rate and diamond quality were evaluated as the input variables were varied. The fixed input variables in the SCD synthesis experiments reported in this Chapter are summarized in Table 7.1, and the variable input experimental variables are listed in Table 7.2. Table 7.1 – Fixed input experimental variables for the SCD synthesis experiments. Fixed input experimental variables Reactor design Reactor B Substrates Single crystal diamond Length of probe (Lp) 3.56 cm Gas chemistry: H2 400 sccm (purity grade: 99.9995%) As listed in Table 7.1, the experiments for demonstrating SCD synthesis in Reactor B employed single crystal diamond substrates. Unless specified the substrates were unused and cleaned, high pressure, high temperature (HPHT) SCD plates that were described in Chapter 4. In order to investigate the deposition on MPACVD synthesized diamond seeds, in a few experiments, the 216 HPHT diamond plates already had a layer of MPACVD synthesized SCD. These seeds were also used as substrates in a few experiments. With or without the synthesized SCD layer, the SCD substrates were cleaned and prepared as described in Chapter 4. Their dimensions varied slightly from one substrate to another. Thus each substrate had to be weighed and measured. Reactor B was already well matched as shown in Chapters 5 and 6. The length of probe, Lp, was kept constant as 3.56 cm for all experiments in this dissertation. The flow rate of hydrogen was held constant at 400 sccm. The substrate holder designs that were used in the SCD synthesis experiments are introduced in Chapter 3 and the details of their configurations are summarized in Appendix A. With the different substrate holder designs, L1, the distance between the top of substrate holder and the bottom of the coaxial section, was varied. As shown in Table 7.2, with the “one-piece” substrate holder, L1 reached a minimum at 52.66 mm, and could be adjusted to maximum at 55.26 mm with a “two-piece” substrate holder design. L2 was varied in the range of 52.1-61.9 mm by adding shims with thicknesses of 0-9.8 mm. Thus the substrate position was varied within the range of -9.24 to +0.56 mm. Ls was also varied to compensate for the variation of L2. Using the tuning process described in Chapter 6, Ls was adjusted within the range of 21.46-21.91 cm. 217 Table 7.2 – The variable input experimental variables for the SCD synthesis experiments. Varied input variables Substrate holder design (the design variations are summarized in Appendix A) Length of short (Ls) 21.46 cm – 21.91 cm Substrate L1 52.66 mm – 55.26 mm position L2 52.1 mm – 61.9 mm Zs Zs = L1 – L2 = -9.24 mm – +0.56 mm CH4 12-28 sccm (purity grade: 99.999%) Gas chemistry 3-7% CH4/H2 N2 (extra) 0-8 sccm (0.9255% N2/H2) 0-200 ppm N2/H2 Pressure (p) 180-280 torr Deposition time 3-30 h Substrate Etching 800-1600 ° C temperature Deposition 950-1400 ° C Microwave power density 200-600 W/cm 3 Output variables Diamond growth rate µm/h Diamond quality Raman, SIMS, transmission, birefringence 218 It is worth noting that the ranges of substrate positions and the lengths of short that are indicated in Table 7.2 were the variable ranges within which the experiments were actually operated. This range could be wider according to the range of L1 and L2 listed in Table 7.2. For example, when L1 was 55.26 mm, and L2 was 52.1 mm, then Zs = L1-L2 = +3.16 mm. However, once the initial investigation of SCD synthesis versus substrate position was completed and the most appropriate substrate position for SCD synthesis was identified, then L2 and Ls were also held constant. Then the substrate position was only adjusted slightly as different substrate holders were employed. The final “best” Zs position was usually negative, i.e. below the z = 0 plane. For example, when L1 was 55.26 mm, L2 was never 52.1 mm, and a substrate position of greater than +0.56 mm was not used. The results of SCD synthesis versus substrate position are displayed in later section of this chapter. The hydrogen and methane input gases had purity grades of 5.5N and 5N respectively and were the same gases that were used in the reactor operating field map measurements presented in Chapter 5. The methane concentration was varied from 3% to 7%. For some experiments, nitrogen gas of up to 200 ppm was introduced as an extra gas input. The nitrogen input gas was 0.9255% nitrogen balanced by hydrogen, with an analytical uncertainty of ± 2% of the nitrogen concentration. When no additional nitrogen was added to the gas input system, the input nitrogen gas impurities were estimated from the known impurities of nitrogen in the hydrogen and methane, plus any additional 219 impurities that arose from any small leaks in the vacuum system. The experimental system used for SCD synthesis was maintained tight, and the leak rate of the reactor that was measured before and after each synthesis experiment was usually less than 1 mtorr/h. For experiments performed without extra nitrogen addition and with a vacuum system leak rate of 1 mtorr/h, the input gas impurities were calculated to be less than 10 ppm. This calculation is given in Appendix B. The reactor was operated in the high pressure regime of 180-280 torr. The substrate temperature was monitored by using an optical emission pyrometer with emissivity of 0.1 during the experiment. As described in Chapter 6 the recorded temperature measurement was an averaged value calculated from a number of measurements for each experiment. After each experimental run, the synthesized samples were cleaned and prepared for further measurement and analysis as described in Chapter 4. In the later sections, the influence of these input variables listed in Table 7.2 on diamond growth rate and quality is discussed. Since the input variables are not completely independent, in order to reduce the interaction of variables, the investigation was carried out by only varying one or two variables for each experiment. In the meantime, no matter how the input variables were varied, the reactor was always operated in the safe and efficient operating regime determined in Chapter 5. Thus while operating in this high pressure regime, a desirable experimental multi-variable process parameter space, i.e. a safe, 220 efficient, robust and high quality SCD process window, was expected to be identified. 221 7.3 Hydrogen plasma etching of the HPHT single crystal diamond substrates During the single crystal CVD diamond synthesis process, the substrate is an essential element which influences the properties of the deposited films, because of the presence of dislocations on the substrate surface or in the bulk. By employing a plasma etching pre-treatment of the substrate surface, most of the dislocations which were originally induced by substrate polishing as well as other defects could be eliminated [52]. Therefore the quality of the grown diamond films can be improved [52, 95, 96] by first employing a pre-deposition plasma etching step. This etching technique also reveals the bulk dislocations that originate from the HPHT substrate during the HPHT crystal synthesis process itself. These dislocations are revealed on the HPHT surface through the formation of etch pits [52] during the etch process. In all the SCD synthesis experiments reported in this dissertation, a one-hour hydrogen-only plasma etching process was employed prior to the deposition process. Using the field map curves the etch process was operated at the same pressure as the deposition process, and was expected to eliminate any existing defects on the substrate surface. Thus this preprocess step was used to prepare the surface for SCD nucleation. Since the deposition process was started immediately after the etching process finished by introducing methane into the feeding gas mixture, the etching results were not 222 routinely evaluated by visually observing the substrate. Thus several individual hydrogen only plasma etching experiments were carried out to demonstrate the effect of etching pre-treatment on the substrate surface. The results of these experiments are described below. 7.3.1 Hydrogen plasma etching experimental techniques The substrates used in the hydrogen plasma etching experiments were unused HPHT diamond seed samples with dimensions of 3.5 mm x 3.5 mm x 1.4 mm. They were etched on both top and bottom surfaces in separate experiments for one hour. A typical SCD synthesis experimental condition that was used is shown in Table 7.3. The operational pressure and hydrogen flow rate were set as 240 torr and 400 sccm, respectively. Ls, Lp and L2 were also held constant as displayed in the table. However L1, and thus Zs were varied by varying substrate holder designs. The substrate temperature in the pre-treatment etching experiments was measured by using the same methods described in Chapter 6. In order to investigate the effect of the etching temperature on the substrate surface, the substrate temperature was adjusted in the range of 800-1600 ° by varying C the substrate holder design and input power, as discussed in Chapter 6. Thus the L1 and Zs were also varied 2 mm as shown in Table 7.3. The detailed experimental conditions for each experiment are summarized in Appendix C. 223 Table 7.3 – Experimental variables used in the hydrogen only plasma etching experiments. Length of short (Ls) 21.6 cm Length of probe (Lp) 3.56 cm Substrate L1 52.87 mm – 54.81 mm position L2 58.6 mm Zs Zs = L1 – L2 = -5.73 mm – -3.79 mm Gas chemistry: H2 400 sccm (purity grade: 99.9995%) Pressure (p) 240 torr Operation time 1h Substrate temperature 800-1600 ° C The diamond etching rate was calculated by using the same methods for diamond growth rate described in Chapter 4. The morphology of substrate surface was observed by using the optical microscope as is described in Chapter 4. 7.3.2 Diamond etching rate versus etching temperature The diamond etching rate as measured by linear encoder versus the substrate temperature is displayed in Figure 7.2. By varying substrate holder 224 designs, the substrate temperature in the etching process was adjusted in the range of 800-1600 ° thus the etching rate also varied. As shown in Figure C, 7.2, the etching temperature range could be divided into three regimes: low etching temperature regime (< 1000 ° medium etching temperature regime C), (1000-1300 ° and high temperature regime (> 1300 ° The etching rate C), C). increased with etching temperature. When the substrate was etched with low or medium etching temperatures, the etching rate was less than 5 µm/h. The etching rate increased dramatically when the etching temperature was higher than 1400 ° As shown in the figure, at high temperatures high etching rates C. of greater than 10 µm/h were obtained. In each SCD synthesis experiment, the growth rate was calculated by using the thickness of sample prior to the etching process as the initial thickness. Taking a typical eight-hour deposition experiment as an example, the one-hour etching process with low or medium etching temperature does not influence the final growth rate calculation much. However, when the substrate was etched with a high substrate temperature, a diamond thickness of approximate 10 µm was etched away in the etching process and would result in a 0.5-1.5 µm/h error in the calculation of the final diamond growth rate. Thus just taking the accuracy of growth rate measurement into account, the etching temperature less than 1300 ° was more desirable. C 225 Figure 7.2 – Etching rate by linear encoder versus substrate temperature in hydrogen plasma etching experiments. (Experimental conditions: 240 torr) 7.3.3 Description of the hydrogen etched substrate surface It was reported that the dislocations on the substrate surface and in the substrate bulk can be revealed [52] by observing the etch pits on the substrate surface. A series of hydrogen only plasma etching experiments were performed and they demonstrated that the size and density of etch pits varied as the substrate temperature increased. The results of these experiments are summarized below. Figure 7.3 shows the examples of optical micrographs of substrate 226 surfaces that were etched by the hydrogen plasma. The three samples were etched on the top surface with substrate temperatures from the three temperature regimes as identified in Figure 7.2 above. The micrographs on the first row show the top view of the entire substrate surface with 25x magnification. As shown in the pictures, as the etching temperature increased, the density of etch pits on the substrate surface increased. The sample which was etched with a high etching temperature displays an exceedingly high etch pits density, and the distribution of the etch pits is more uniform than the distributions of the samples etched with low or medium temperature. In Figure 7.3 a small part of the substrate surfaces is zoomed in with magnification of 500x as are displayed in the pictures on the second row. It is clear that the size of the etch pits increased from less than 1 to a few µm, to sizes larger than 10 µm as the etching temperature is increased. The micrographs on the third row are a side view of the substrate surface. The samples etched with low or medium temperature do not show etch pits on the side surfaces of the substrates. Etch pits and traces were only observed on the sides of the samples that were etched at high temperatures. The outline of the high density etch pits on the top surface can also be seen in the side view micrographs. 227 Figure 7.3 – Examples of surface morphologies from samples which were etched with low, medium, and high etching temperature. The pictures on first row are the top view of the substrate surface. The pictures in the second row are “zoomed in” photos from a portion of the substrate surface as indicated by the square located on the substrate surface in the photos on the first row. The pictures on third row are side views of the substrate surface. 228 The density and size of etch pits were measured directly from these micrographs. Figure 7.4 shows examples of the measurement. On the optical micrographs of the top view and the side view of the substrates, the dimensions of the etch pits could be measured by an image processing computer program (Image Pro plus 5.1). The measurements of the etch pits indicated by the arrows are displayed in the figure. Figure 7.4 – Examples of measurement of size of etch pits: (a) top view of sample 44; (b) side view of the deposition edge of sample 45. According to the observation of the micrographs of the substrates, the hydrogen plasma etching was very mild at low etching temperatures. The etch pits density was not uniform, as shown in the pictures on the left column of Figure 7.3. The average of etch pits density was counted to be approximately 9 -2 10 m , and the etch pits dimensions were usually less than 1 µm. When 229 etching temperature was in the medium and high etching temperature regimes, the size and density of etch pits both increased. The etch pits density of 10 m -2 could be obtained for substrate temperature higher than 1000 ° C, 10 and there is a pattern for the etch pits distribution as is shown in the middle picture of Figure 7.3. In the high etching temperature regime, the etching on substrate surface was more intense than at lower temperature. The etch pits size would increase from around a few microns to a few tens microns, even though the etch pits density remained around 10 10 -2 m . The plasma etching on the substrate surface reveals the bulk dislocations that reach the surface through the formation of etch pits. It was believed that the etch pits density could be used as a criterion to select the least defective substrate [52]. However, the observations described above show that the etch pits density also depend on the etching temperature. Since the results of plasma etching experiments were repeatable versus etching temperature, the size and density of etch pits mostly represented the etching temperature, other than the defects density in the substrate. When using the higher etching temperatures, the etching rate was greater, thus more diamond was etched away, etch pits were deeper in the bulk, and then more dislocations were revealed. Both the top and bottom surfaces of the HPHT seed substrate samples were etched in separate experiments. By comparing the morphology of the two surfaces of each sample, it was confirmed that the hydrogen plasma etching 230 process revealed the growth sectors in substrates. As shown in the pictures of Figure 7.3, there are patterns displaying on the etched substrate surface due to the non-uniform etch pits densities. The difference of etch pit density distribution revealed the different HPHT growth sectors [97]. As discussed in Chapter 2, the HPHT diamond substrates were obtained from larger HPHT crystals containing several growth sectors (shown in Figure 2.29). Thus the top and bottom surfaces of a seed substrate that were identified in Chapter 4 were not equivalent. The results of the etching experiments presented in this section show that the top surface of most samples was composed mainly of a single <100> growth sector, while the bottom surface was made up of several different sectors. Figure 7.5 shows some examples of the revealed growth sectors by hydrogen plasma etching. No matter what the etching temperature was, or what the etch pits density was, the HPHT growth sectors could be highlighted by the spatial distribution of the various etching pits densities. The position and size of various growth sectors are clearly shown on the surface, which exhibit large differences in the impurity concentrations across the seed substrate surface [52, 97]. Mostly the growth sectors were shown in the bottom surface of the substrate. There was only one exception. As enclosed by the dashed line in Figure 7.5, the top surface of sample 45 reveals the growth sectors, rather than the bottom surface. However, in all SCD synthesis experiments reported in this dissertation there was no chance to check the growth sectors 231 prior to deposition process, and thus the top surface of HPHT diamond substrates were always used as the deposition surface. Figure 7.5 – Examples of revealed growth sectors in HPHT diamond substrates. Usually the bottom surface of the substrate samples revealed the various growth sectors. However sample 45, which is shown enclosed above, is reversed. It was the only sample found to exhibit this reverse behavior. 232 7.3.4 Summary In order to study the influence of plasma etching on the substrate surface, a series of one-hour hydrogen-only plasma etching experiments were carried out on HPHT diamond substrates. Most of the experimental conditions, such as Ls, Lp, L2, pressure, hydrogen flow rate, and operation time were held fixed. The etching temperature was the most important variable in the investigation. It was adjusted by varying the substrate holder designs, which then also resulted in small associated variations in L1 and Zs, and absorbed power. The etching temperature was then able to be varied over the range of 800-1600 ° C. The experimental results show that the hydrogen plasma etching process could remove the defects on the substrate surface and reveal the dislocations in the seed substrate. However after SCD synthesis process the dislocations in the crystal bulk did not affect the diamond growth, thus the hydrogen plasma etching process could ensure that there are no defects in the center of the synthesized diamond. The bottom surface of HPHT diamond substrate was usually made up of several growth sectors, which was also be revealed by the etching pre-treatment process. The etching rate, etch pits density and etch pits dimensions all increase when the etching temperature was higher. At high etching temperatures, i.e. greater than 1300 ° the etching on the substrate surface was increased. The C, 233 etching rate could reach 10 µm/h, and etch pits with sizes of 20 µm were observed. The edges of substrate were etched as far down from the top or bottom surface as 100 µm. The results of hydrogen etching experiments in various temperature regimes are summarized in Table 7.4. For all SCD synthesis experiments performed in this thesis, the top surface of HPHT diamond substrate was chosen as deposition surface due to the high possibility of presence of a single <100> growth sector. Considering the growth rate calculation accuracy and diamond sacrifice in the etching process, the medium etching temperature with the lower etching rate was more desirable. Due to the associated high deposition temperature and the poorly synthesized diamond quality, the high etching temperature was not preferred. This is described in Section 7.4. Table 7.4 – Results of hydrogen plasma etching experiments. Low etching Medium etching High etching temperature temperature temperature Etching temp. < 1000 ° C 1000-1300 ° C > 1300 ° C Etching rate < 2 µm/h < 4 µm/h 6-13 µm/h Etch pits density ~ 10 m Etch pits size < 1 µm 0.5-5 µm 1-25 µm No observation No observation 20-40 µm 9 -2 ~ 10 10 m -2 ~ 5x10 10 (top & bottom) Side etch pits size 234 m -2 7.4 Experimental evaluation of single crystal diamond synthesis in multi-variable space The experimental multi-variable parameter space that was investigated in this dissertation for SCD synthesis is described briefly in the section 7.2. In order to identify a safe, efficient, and robust experimental process window in the high pressure regime for production of high growth rate high quality SCD, the influence of those important experimental input variables on the diamond synthesis process was studied. CVD diamond deposition is a complex process. Thus it was necessary to simplify the problem by varying one variable at a time while keeping other variables constant at a time. Using the reactor road map curves described in Chapter 5, the SCD synthesis experiments performed in this dissertation were operated under the operationally safe and microwave coupling efficient experimental conditions. The SCD synthesis experimental results were evaluated by measuring the diamond growth rate, and characterizing a number of diamond quality measures such as substrate surface morphology, Raman spectroscopy, secondary ion mass spectrometry (SIMS), etc. The role of each single experimental variable in SCD deposition process is explored by carefully analyzing the experimental results. It was expected that while operating at high pressures (>180 torr) a safe, deposition efficient and high quality high growth rate window would be identified for SCD synthesis. 235 The details of the conditions for each experimental run reported in this section are summarized in Appendix D. 7.4.1 Diamond growth versus the variation of substrate position As mentioned in previous chapters, the reactor used in the SCD synthesis experiments, Reactor B, was always operated in a well matched condition. In order to achieve the TM013 + TEM001 hybrid mode electromagnetic excitation and excellent matching, the reactor tuning variables, Ls, Lp, L1 and L2, had to be adjusted to the proper lengths. In the experiments reported in this dissertation the length of probe, Lp, was usually kept constant, at 3.56cm. During the exploratory experiments Ls, L1 and L2 were adjustable. However the length of the short, Ls, was only readjusted to achieve a microwave match when L2 was varied. As already described in Chapter 6 the “microwave matched” position Ls depended on L2, and thus Ls was varied as L2 was varied. The substrate position, Zs, the distance between the top of substrate holder and the z = 0 plane, was defined as L1 - L2. Thus in the experimental run reported in later sections when Lp, L2 and Ls were fixed, L1 was adjusted by varying substrate holder designs. Then the associated varied substrate position, Zs, could represent the tuning condition of the reactor for the experimental run. However, in the exploratory experimental run described in this section, in order to obtain large range of variation, Zs was adjusted by 236 only varying L2, and Lp, L1 were held fixed. Ls was varied slightly to compensate the variation of L2 as well. The multiple adjustments of Ls, L1 and L2 allowed one to simultaneously match the reactor and still control the discharge behavior, such as the size, shape, position and power density of the discharge. Additionally while operating at a constant pressure the substrate temperature and the discharge power density could also be varied. The process control via substrate position variation, i.e. the variation of substrate temperature and the discharge power density versus substrate position is presented in Section 6.5 (see Figure 6.9). In the SCD synthesis experiments reported in this dissertation, as described in section 7.2, the substrate position could be varied from +1 mm to -10 mm. When the substrate position is varied, the electric field, the discharge shape and position are varied in the space above and around the substrate. This has been discussed in earlier publications. See for example references [68, 86, 90]. The variation of electric field in the reactor versus Zs can be understood using the results of numerical calculations of the electromagnetic field patterns in an empty (no discharge) Reactor B. Figure 7.6 shows a series of such numerical calculations that display how the reactor electromagnetic field patterns inside the reactor vary as Zs is varied. In particular the results show how the normal component of the electric field above the substrate holder 237 varies as Zs is varied from +4 mm to -5 mm. The reactor cavity is cylindrical and thus the problem is phi symmetric. Therefore the simulation of the electric field was only applied to the 2-dimensional coaxial plane with eigenfrequency analysis using software COMSOL Multiphysics. The model included all real details of probe, base plate, substrate holder, bell jar and quartz tube as shown in the Figure 3.12. Lp was set at 3.5 cm, and L1 was also kept constant at 5.65 cm. As Zs varied, the eigenfrequency was adjusted to be always kept as close as possible to the excitation frequency of 2.45 GHz. This was done by varying Ls in a fashion similar to the matching process of a real-life experiment. As shown in Figure 7.6, when Zs was adjusted from above to below the z = 0 plane in steps of 1 mm, the electric field above the substrate increased, and became more focused and more intense. On the other hand, the electric field underneath the substrate also increased when Zs decreased from +4 mm to -5 mm as is shown in the numerical simulation results in Figure 7.6. The high intensity of electric field in the coaxial region could cause problems during the SCD synthesis experimental run, such as quartz tube overheating or discharge breaking down. Thus during an experimental run it was necessary to choose a range of substrate positions where the high discharge power density deposition process was enabled and the reactor could be operated safely. Note that this simulation does not include the discharge. Once the discharge is present all the electric field intensities in the reactor are reduced. Then the electric field in the coaxial region is also reduced and the chance of 238 discharge breaking down is greatly reduced even though the relative spatial electric field pattern remains approximately same. Figure 7.6 – The numerical simulation results of variation of electric field inside the reactor versus Zs for Reactor B (no discharge). 239 Figure 7.7 shows an example of the actual experimental results, i.e. the growth rate and the substrate temperature, versus substrate position. The experiments were carried out at pressure of 240 torr. The deposition time was 8 hours for each experimental run. The methane concentration was 5%, and L1 was kept constant at 52.66 mm by using a same substrate holder in all of the experiments. Figure 7.7 – Growth rate and substrate temperature versus Zs for Reactor B. Experimental conditions: pressure = 240 torr, CH4/H2 = 5%, L1 = 52.66 cm, input power = 2.2 kW. 240 In the exploratory experiments, L2 was varied from 52.1 cm to 61.9 cm by adding circular stainless shims with thicknesses up to 9.8 mm. As each shim was added the length of short, Ls, was adjusted to achieve excellent matching, as has been described in Chapter 6. Thus Ls was varied from 21.46 cm to 21.91 cm in the experiments presented in Figure 7.7. As shown in Figure 7.7, when the substrate position was varied from +0.56 mm to -9.24 mm, the diamond growth rate, represented by black square data points in the figure, increased from less than 4 µm/h to around 11 µm/h, while the substrate temperature shown as red circle data points was in a range of 930-1000 ° For all the experimental run in Figure 7.7 the input C. power was kept around 2.2 kW. When substrate position is negative, the substrate temperatures are very close, with a variation of less than 10 ° As C. presented in Section 6.5 (see Figure 6.9), the discharge power density also increases with substrate temperature when substrate position decreases. As shown in Figure 7.7, due to increasing discharge power densities, when substrate position decreased from -2.67 mm to -9.24 mm, the diamond growth rate increases almost by factor of 2. As discussed above, when substrate position was negative, the electric field above the substrate became more focused and intense. This enabled the higher discharge power densities and growth rates. However this also increased the possibility of quartz tube heating and discharge breaking down. 241 At large negative position value for Zs, the discharge might not be stable and jump down below the substrate and attach itself to the quartz tube below the substrate. In practice, there were several quartz tube heating accidents that occurred when shims with thickness of 8.1 mm (Zs = -7.54 mm in Figure 7.7) were used. This means that the operation under these conditions was out of the safe and efficient reactor operating regime. The selection of substrate position influenced both the growth rate, as well as the process safety. Thus for the experiments performed in this dissertation the safe and efficient operating regime for Zs was limited between -6 mm and -3 mm. According to experimental results and taking safety into account, the 6.5 mm thick shim (Zs = -5.94 mm in Figure 7.7) was chosen as the appropriate substrate position SCD synthesis. Thus L2 was kept constant at 58.6 mm for approximately 80% of the experiments reported in the rest of this dissertation. Ls was also fixed at 21.6 cm in these experiments. Since substrate holder designs were varied (see Appendix A), L1 was not fixed. Z s could be varied from -3.34 mm to -5.94 mm by varying the substrate holder design and hence by varying L1. 242 7.4.2 Diamond growth with nitrogen addition It is widely understood that the addition of a small amount of nitrogen would strongly increase the diamond growth rate [20, 43, 46, 93, 94]. Thus a set of SCD synthesis experiments were performed in Reactor B that investigated the influence of the amount of the nitrogen concentration in the gas phase on SCD synthesis. Thus a number of experiments were carried out to determine the relationship between the nitrogen concentration in the gas phase and the diamond growth rate and the diamond quality. The quality of deposited diamond film was characterized by Raman spectroscopy, and secondary ion mass spectrometry analysis (SIMS). The morphology of the substrate surface was also observed by using an optical microscope. Figure 7.8 displays the diamond growth rate versus the additional nitrogen concentration in the gas phase. The experiments operated by Reactor B were performed at 220 and 240 torr. Several experimental variables were held constant: CH4/H2 = 5%, Ls = 21.46 cm, L1 = 52.66 mm, L2 = 60.2 mm, Zs = -7.54 mm. In order to reduce the varied parameters, the substrate temperature was tried to be controlled within the range of 1080-1120 ° C. However, for the experiments with high nitrogen concentration in the gas phase, the substrate thickness increased quickly versus time due to the high growth rate. Thus during the experimental runs the substrate temperature also increased rapidly versus deposition time. Within the process regime limits of absorbed power (see the field map in Figure 5.7), it was difficult to control the 243 substrate temperature at the lower temperature level. Therefore the substrate temperature in experiments with 100 ppm additional nitrogen concentration in the gas was an exception which was around 30 ° higher than 1120 ° The C C. error bar of 10 ppm for data points at 0 ppm additional nitrogen concentration indicates the possibility of nitrogen impurities of less than 10 ppm present in the reactor when there was no additional nitrogen introduced. The calculation of nitrogen impurity concentration is presented in Appendix B. In Figure 7.8, the triangle green data points represent the growth rate at 240 torr, and the circle red data points are the growth rates at 220 torr. They are compared with the square black data points which were obtained for Reactor A operating at 160 torr with a 7% methane concentration [101]. As expected at each pressure, the growth rate increased as the nitrogen content in the gas phase increased. For a particular percent nitrogen concentration addition, the slope of the curve is higher for the higher pressure operation than the lower pressure operation. That is, the growth rate increased faster versus N2 addition at the higher pressures than it did at lower pressures. This increase in the growth rate versus N2 addition in the gas phase at high pressure is probably due to the higher discharge power density and higher gas temperatures at the higher pressures. The higher gas temperatures at high pressures may be dissociating the added N2 more completely. This for a given amount of N2 input there are more atomic N species available in the gas phase at high pressures. 244 Figure 7.8 – Diamond growth rate versus additional nitrogen concentration in the gas phase for Reactor A and B. Experimental conditions for Reactor B: CH4/H2 = 5%, Zs = -7.54 mm. The experimental data for reactor A was from ref. [101]. A. Tallaire et al. also reported that the addition of nitrogen even at rates as low as a few ppm in the gas phase was enough to increase the growth rate [45, 46, 57]. Figure 2.26(a) shows that with 6 ppm and 10 ppm additional nitrogen in the gas phase, the growth rate was increased from 6 µm/h to 10.3 µm/h and 15.8 µm/h, respectively (experimental conditions: 165 torr, 4% CH4/H2) [57]. Since the nitrogen impurities in the gas phase were claimed to 245 be less than 0.5 ppm in their experimental setup when no extra nitrogen was added, the growth rate of 10-15 µm/h was comparable with our results at 0 ppm additional nitrogen concentration (with less than 10 ppm nitrogen impurities in the gas phase). A maximum deposition rate of 55 µm/h was reported by introducing 200 ppm of nitrogen at 875 ° C (experimental conditions: 165 torr, 4% CH4/H2) [46]. Compared with the growth rate of 47 µm/h with nitrogen content of 100 ppm shown in Figure 7.8, the operational pressure of their results was lower but the nitrogen concentration was much higher. The reported substrate temperature from their work was generally 200-300 ° lower than that obtained in our experiments. The discharge power C 3 density of 95 W/cm at 150 torr [97] of their reactor was almost equivalent as the power density of Reactor B at 100 torr due to different measurement methods. In a word, in despite of the different experimental conditions, the influence of additional nitrogen on the diamond growth rate which is presented in this section followed a similar trend versus nitrogen variation with results which have been reported earlier. The quality of the synthesized SCD was determined from Raman FWHM and SIMS measurements. The nitrogen content in the synthesized diamond crystal as measured by SIMS, and the Raman FWHM as measured on the substrate surface are plotted in Figures 7.9 and 7.10. The data points in these two figures are from a same set of experiments. The experimental conditions are: pressure = 240 torr, CH4/H2 = 5%, Ls = 21.6 cm, L1 = 52.87 246 mm, L2 = 58.6 mm, Zs = -5.73 mm. The substrate temperature was controlled and was held within the range of 1280-1330 ° The total nitrogen content in C. the gas phase in these two figures is the sum of any nitrogen impurities from any system leaks, plus impurities from the input gases (H2 and CH4) and any additional nitrogen added from the separate input nitrogen/hydrogen gas mixture. When the additional nitrogen content was zero the nitrogen impurities content in the gas phase was assumed to be 10 ppm or less. In Figure 7.9, the square black data points represent the diamond growth rate. The circle red data points in Figures 7.9 and 7.10 are the nitrogen content in the synthesized diamond as measured by SIMS. The black data points in Figure 7.10 are FWHM of Raman spectra. There are two horizontal dashed lines in Figure 7.10, which display Raman FWHM data from two reference diamond samples. The lower dashed line represents the Raman FWHM of a type IIIa SCD sample from Element Six and the upper dashed line represents the Raman FWHM of a typical type Ib HPHT seed that was used in the experiments. These Raman FWHM measurements were made using the same Raman instrument as all the other Raman FWHM measurements and thus are listed in the Fig. 7.10 as benchmark FWHM references for vey high quality diamond and for diamond with nitrogen impurities; i.e. type Ib diamond. The measured Raman FWHM for the type IIIa diamond sample was 1.57 cm -1 and the Raman FWHM for the HPHT seed was 1.88 cm . 247 -1 Figure 7.9 – Growth rate and nitrogen content in crystal versus total nitrogen concentration in the gas phase. Experimental conditions: pressure = 240 torr, CH4/H2 = 5%, Zs = -5.73 mm. As shown in Figures 7.9 and 7.10, when nitrogen impurities of up to 200 ppm are introduced in the feed gas, the diamond growth rate increases, the nitrogen incorporation in the crystal increases, and the diamond quality decreases. The SIMS analysis of the synthesized SCD indicates that for diamond that is synthesized with a total input nitrogen gas phase concentration of less than 10 ppm there is less than 300 ppb (below the detection limit of the SIMS measurements as described in Chapter 4) of both nitrogen and silicon in 248 the synthesized diamond. The N content of 300 ppb was indicated as the blue dashed line in this figure. As also shown in Figure 7.9, the FWHM of the Raman spectra of this diamond is less than 1.7 cm cm -1 -1 and is close to the 1.57 that was measured for the type IIIa diamond. When the additional nitrogen content in gas phase is increased to 200 ppm, the nitrogen content in the synthesized diamond increases to around 4.5 ppm, and the Raman FWHM -1 is over 1.9 cm . This value is higher than the FWHM for the typical HPHT diamond substrate. Figure 7.10 – Raman FWHM and nitrogen content in the crystal versus total nitrogen concentration in the gas phase. Experimental conditions: pressure = 240 torr, CH4/H2 = 5%, Zs = -5.73 mm. 249 The morphology of synthesized diamond versus additional nitrogen content was observed by using an optical microscope. The micrographs of the synthesized diamond samples whose experimental results are shown in Figures 7.9 and 7.10 are displayed in Figure 7.11. As shown in Figure 7.11(a), with substrate temperature higher than 1250 ° C, the synthesized diamond surface did not show any visual steps, but exhibited a smooth and “bulky rock” like appearance. No unepitaxial defects were formed in the substrate surface. A similar morphology was observed for other diamond substrates that were synthesized under high substrate temperatures and with 0 ppm extra nitrogen gas phase addition. This is presented in later section. As nitrogen gas phase concentration of 25 ppm was added, the Figure 7.11(b) shows that the substrate surface started roughing though it had similar appearance to the sample with 0 ppm nitrogen addition. As additional nitrogen concentration continued to increase, the step-bunching appearance was observed as shown in Figure 7.11(c)-(h). It was known that the advancing growth steps could be blocked by adsorbed impurities such as nitrogen, leading to the bunching phenomena and the formation of macro steps [95, 96]. The “bulky rock” like appearance disappeared. According to the close up micrographs with 100x magnification in Figure 7.11(d), (f) and (h), the steps gradually become narrower and thinner as additional nitrogen concentration increased. A polycrystalline diamond rim occurred when a nitrogen concentration of 150 ppm was added (Figure 7.11(e)). As shown in Figure 250 7.11(g), when the nitrogen addition reached 200 ppm, there are more defects on the substrate surface. The polycrystalline rim also becomes larger as more nitrogen is added. This might be one of the reasons for the high Raman FWHM value as was shown in Figure 7.10. J. Achard et al. [45, 46] also reported the step bunching phenomenon on substrates with nitrogen addition in gas phase of 75-200 ppm. However, the large square pyramidal features and the variation of the size and shape of those pyramidal features that they reported were not observed in our results. In conclusion, these SCD synthesis experiments with nitrogen addition demonstrated the sensitivity at high pressures of the diamond growth rate to the nitrogen concentration in the gas phase. Though high growth rates of 47 µm/h were achieved at 240 torr with nitrogen addition, the diamond quality deteriorated versus the addition of N2. Since a goal of this dissertation research was to synthesize high quality diamond, the method of boosting up diamond growth rate by introducing nitrogen impurities was not investigated any further. The experiments presented in later sections were performed without extra nitrogen addition in the gas phase. Thus in these experiments the N2 addition in the gas phase was limited to less than 10ppm. 251 Figure 7.11 – Micrographs of the surfaces samples grown with different additional nitrogen concentrations and substrate temperatures. Experimental conditions: pressure = 240 torr, CH4/H2 = 5%, Zs = -5.73 mm. Rtm is the average distances between the surface peaks and valleys. 252 7.4.3 Diamond growth rate versus simplified multi-variable space 7.4.3.1 Introduction As introduced in section 7.2 the optimization of single crystal diamond deposition using Reactor B is a multi-variable problem. Figure 7.12 shows the block diagram of the experimental variables that are simplified from the variables that are shown in Figure 7.1. Because as discussed in the previous section diamond synthesis with additional nitrogen present in the gas phase results in lower quality SCD, in the following experiments the impurity nitrogen concentrations in the gas phase were experimentally controlled to the very low input level of less than 10 ppm. Additionally once an appropriate L2 was determined then the length of short, Ls, was also fixed. Thus in the experiments reported in this section these two input variables were kept constant. As shown in the left block on the middle row in Figure 7.12, the “N2/H2” and “reactor tuning: Ls” are in grey and crossed over. However, there are still many other important experimental variables that influence the diamond synthesis process. In order to explore the SCD deposition process further, it was necessary to further simplify the problem. As shown in the blocks on the third row in Figure 7.12, the group of input variables was further reduced to two important variables: pressure, and methane concentration. Other input variables, such as absorbed power, substrate position, and substrate holder designs, are all related with internal variables: 253 substrate temperature and discharge power density. Figure 7.12 – Block diagram of experimental variables simplified for SCD synthesis process. As described in section 7.4.1, when L2 was constant, substrate position depends on substrate holder designs. In a word, the substrate temperature and discharge power density are functions of pressure, absorbed power and substrate holder designs, which could be controlled by the many methods that are presented in Chapter 6. Thus the SCD synthesis experiment outputs, growth rate and diamond quality, which are functions of input variables could 254 be simplified as functions of pressure, methane concentration, substrate temperature and discharge power density. This section explores each of these variables’ influence on SCD growth rate in this simplified multi-variable space. It is worth noting that the input variables and internal variables are all interdependent. 7.4.3.2 Growth rate versus substrate temperature First the dependence of SCD growth rate and substrate temperature was investigated. It was well known from lower pressure experiments [66, 89] that the growth rate was dependent on the substrate temperature. Thus a series of experiments of SCD growth rate versus substrate temperature were performed on Reactor B and the results are displayed in Figures 7.13 and 7.14. For data displayed in Figure 7.13, the operational pressure was held constant at 240 torr and the input gas chemistry was CH4/H2 = 6%. Most reactor geometrical parameters in these experimental runs were fixed: Lp = 3.56 cm, Ls = 21.6 cm, L2 = 58.6 mm. The variation of substrate temperature shown in the figure was achieved either by (1) adjusting the substrate holder designs before each run and/or (2) by slightly varying the input power during the run as described in Chapter 6. During these deposition experiments the absorbed power was also slightly adjusted versus time in order to maintain an approximately constant substrate temperature versus time. This adjustment 255 process is described in Chapter 6 as well. The reactor operating conditions and the variation of absorbed power were located within the efficient and safe experimental operating regime that is described in Chapter 5. As the substrate holder designs were varied to adjust the substrate temperature, the substrate position, Zs, was also varied slightly from run to run within the range of -3.34 mm – -5.73 mm. The deposition time for each experiment was kept constant at 8 hours, except for one data point that is shown in Figure 7.13 that has a substrate temperature of 1170 ° and has the highest growth rate of 41.01 C µm/h. This experiment had a deposition time of 7.5 hours. Figure 7.13 – SCD linear growth rate versus substrate temperature for Reactor B. Experimental conditions: pressure = 240 torr, CH4/H2 = 6%, L2 = 58.6 mm, 256 Ls = 21.6 cm, Zs = -3.34 mm – -5.73 mm. The data in Figure 7.13 clearly show that at 240 torr and with a methane concentration of 6% there is a broad high growth rate SCD deposition window between 950 and 1300 ° C. As shown the growth rate data points do not scatter randomly in the substrate temperature range. The green dashed curve displays the approximate, experimentally observed SCD growth rate variation versus substrate temperature. There is a growth rate maximum versus temperature in this substrate temperature range. The maximum growth rate is a factor of 3 larger than the minimum value. The observation of the SCD growth window was further explored by varying the methane concentration. Figure 7.14 displays the experimental results of growth rate versus substrate temperature as the methane concentration is varied in steps from 4-7%. The pressure was still held constant at 240 torr. The other experimental variables were kept the same as the variables in the experiments shown in Figure 7.13 with methane concentration of 6%. The deposition time for experiments with methane concentration of 5% was 8-10 hours. For other experiments, the deposition time was held constant at 8 hours. 257 Figure 7.14 – SCD linear growth rate versus substrate temperature for different methane concentrations. Experimental conditions: pressure = 240 torr, CH4/H2 = 4-7%, L2 = 58.6 mm, Ls = 21.6 cm, Zs = -3.34 mm – -5.73 mm. As shown in Figure 7.14, for different methane concentrations the SCD growth rates were scattered within a similar SCD growth window of 950-1300 ° The growth rate increased as methane concentration increased. C. At each constant methane concentration the growth rate variation was similar to that observed for 6% methane shown in Figure 7.13. In Figure 7.14, the green, red and black dashed lines are roughly drawn to indicate the approximate diamond growth rate versus the substrate temperature for 258 methane concentrations of 6%, 5% and 4%, respectively. The amount of experimental data obtained with methane concentration of 7% was not large enough to define a curve. Within the growth rate window there is clearly a maximum growth rate versus substrate temperature for each methane concentration. The simple curves as drawn in this figure indicate the general trend of growth rate versus substrate temperature. As shown in Figure 7.14, the maximum growth rate occurs within the substrate temperature range of 1100-1200 ° and for a constant operational pressure the maximum appears C, to shift slightly upwards as the methane concentration is varied. J. Achard et al. also investigated the variation of growth rate versus substrate temperature at a constant nitrogen concentration [46]. Three SCD growth experiments were performed at 800 ° 875 ° and 950 ° with the C, C, C addition of 100 ppm of nitrogen in the gas phase (experimental conditions: 165 torr, 4% CH4/H2). The growth rates as a function of substrate temperature were reported. Growth rates reached a maximum at 875 ° C and then decreased when the temperature was further increased up to 950 ° These C. results exhibit a similar variation versus temperature to our results. However it is useful to note that the substrate temperatures from their experiments were shifted 200-300 ° C lower than ours, and the experiments were performed under the condition of 100 ppm nitrogen addition in the gas phase. Additionally the amount of their experimental data was not large. 259 7.4.3.3 Growth rate versus pressure Figure 7.15 specifically displays examples of the growth rate versus pressure for Reactor B. All data displayed in Figure 7.15 were collected from the experiments using the same substrate holder and thus the substrate position was kept constant at -4.49 mm. According to the results from the investigation of growth rate versus substrate temperature presented above, in order to reduce the effect of substrate temperature on the growth rate the SCD synthesis experiments versus pressure were arranged to be performed within the narrow range of 1050-1080 ° substrate temperature. However, since the substrate position C was fixed, the substrate temperature was only adjusted by varying input power. When the reactor was operated in the safe and efficient experimental operating regime which is determined in Chapter 5 (see Figure 5.7), the ranges of input power for safe and efficient operation and associated substrate temperature were limited. Thus there was not much room for the variation of input power within the substrate temperature controlling process. For experiments at 180 torr in Figure 7.15, the substrate temperatures were only obtained within the range of 970-1000 ° As pressure increased, C. the safe and efficient experimental operating regime moved to higher temperature/power space, and the substrate temperature was successfully controlled in the narrow range of 1050-1080 ° C. 260 When pressure was 280 torr, the discharge became more intense and freely floating, and the “discharge blinking” phenomenon due to the pulsing of input power mentioned in Chapter 5 was more obvious. The discharge was not able to be maintained reliably at these low input power levels as the experimental conditions were adjusted to obtain the low substrate temperatures. As is shown in Figure 7.15, the experiment of growth rate of 36.07µm/h and methane concentration of 6% at 280 torr was performed with a substrate temperature of 1072 ° and was only maintained for 5.45 hours before the C, discharge went out. Thus for experiments at 280 torr the substrate temperature was controlled in a bit higher temperature range. The experimental data with a growth rate of 22.87 µm/h and a methane concentration of 5% at 280 torr in this figure was obtained with a substrate temperature of 1097 ° C. In addition, the experimental deposition time for a methane concentration of 5% at 200 torr was 9.5 hours. For other data points in Figure 7.15, the deposition time was kept constant at 8 hours. 261 Figure 7.15 – Growth rate versus pressure for different methane concentrations for Reactor B. Experimental conditions: pressure = 180-280 torr, CH4/H2 = 5-6%, Ls = 21.6 cm, L1 = 54.11 mm, L2 = 58.6 mm, Zs = -4.49 mm. As shown in Figure 7.15, the SCD growth rate increases as pressure increases, no doubt due to the high and increasing microwave power densities at high pressures. Figure 7.16 shows the same SCD growth rate data with methane concentrations of 5-6% versus microwave power density which were obtained within the operating field map measurements presented in Chapter 5 (see Figure 5.9). For example, using 6% methane concentration in the gas phase, as pressure increased from 180 torr to 280 torr, the microwave power 262 density was increased by a factor of 2.6, and the SCD growth rate increases 3 times. Similar to the results from the section above, Figure 7.15 and 7.16 also show that the growth rate increases as methane content is increased. Figure 7.16 – Growth rate versus microwave power density for different methane concentration for Reactor B. Experimental condition: pressure = 180-280 torr, CH4/H2 = 5-6%, Ls = 21.6 cm, L1 = 54.11 mm, L2 = 58.6 mm, Zs = -4.49 mm. It is worth noting that when the experiments were performed under similar conditions the SCD growth rate increases fast from 180 torr to 260 torr and then decreased a bit at the pressure of 280 torr. 263 This phenomenon might be explained by the shifting of the curves of diamond growth rate versus substrate temperature as the pressure varied. Substrate temperature and pressure are interdependent variables as mentioned in the beginning of this section. The dependence curves of SCD growth rate versus substrate temperature described above (see Figure 7.13 and 7.14) were determined only for a certain pressure of 240 torr. As presented in Chapter 5, the safe and efficient reactor experimental operating regime for each pressure moves to higher substrate temperature ranges as pressure increases. Similarly it could be assumed that the growth rate versus substrate temperature curves move to higher substrate temperature ranges as pressure increases as long as the reactor is operated within the safe and efficient experimental operating regime and these curves are similar. Though this assumption of growth rate versus substrate temperature was not verified yet at pressures other than 240 torr in our work, the similar trends for growth rate versus substrate temperature at low pressures (<180 torr) were observed and reported earlier for polycrystalline diamond deposition [66, 108]. The shifting of the growth rate versus substrate temperature curves with this assumption is displayed in Figure 7.17. 264 Figure 7.17 – Shifting of the curves of growth rate versus substrate temperature as pressure varies. As shown in Figure 7.17, there are three growth rate versus substrate temperature curves 1, 2, and 3, which are obtained at pressures of p1, p2 and p3, respectively. p1 is higher than p2, and p3 is the lowest pressure. As assumed above, the curve 1 is placed in the top right which represents the highest pressure, growth rate and substrate temperature region, and curve 2 is a bit lower and on the left. The pressure, growth rate and substrate temperature region of curve 3 at p3 are lowest. As is showed between the two vertical dashed lines in Figure 7.17, when the experiments were performed in 265 the narrow range of substrate temperatures, say between Ts-Ts’, the experimental results are indicated by points A, B and C. The growth rate of point A is lower than the growth rate of point B, though A was performed at higher pressure of p1 than B at pressure of p2. Due to the lowest operating pressure, the growth rate of point C is the lowest among the three data points. In a word, when the growth rate versus substrate temperature curves shifted as pressure varied, a substrate temperature which could result in maximum growth rate for a lower pressure might only locate in the low growth rate range of the curve for a higher pressure. Thus the phenomenon that with similar substrate temperatures the growth rate at lower pressure is higher than the growth rate at higher pressure is possible. This implies that even higher growth rates may be able to be achieved if the rector was redesigned to synthesize diamond with even higher substrate temperatures at higher pressures. Generally, as pressure increased the SCD growth rate increases due to the higher microwave power densities at higher pressures. As shown in Figure 2.26(b), A. Tallaire et al. reported growth rates as a function of power densities for methane concentrations of 4% and 7% as well [57, 97]. At substrate temperature of 850 ° and no additional nitrogen in the gas phase, the growth C rate of 8 µm/h and 18 µm/h were obtained at pressure of 200 torr and 3 microwave power density of 125 W/cm . These results are almost equivalent to the growth rates from our results at 180 torr and microwave power density of 3 200 W/cm . However, A. Tallaire et al. used a metallic-type reactor for 266 experiments, which limited the operations at high power densities and high pressures because the reactor chamber walls could be overheated by electromagnetic joule heating on the metallic reactor walls. Additionally, the measurements of microwave power density and substrate temperature of this group are very different from the methods that were used in this dissertation research. However there are similarities between the results from our groups, but the two results cannot easily be compared. 7.4.4 Diamond morphology versus simplified multi-variable space The morphology of the synthesized diamond surface was observed visually by using an optical microscope. In order to synthesize diamond at high growth rates with a flat surface with no unepitaxial crystallites, it was required to explore the influence of pressure, methane concentration and substrate temperature on the diamond morphology. The experimental investigation of diamond morphology followed a similar set of activities as were employed for the investigation of SCD growth rate that was presented in the previous section. Figure 7.18 shows several examples of the micrographs of the synthesized diamond surface versus substrate temperature. The synthesized SCD samples were selected from the experimental results displayed in Figure 7.13. The experimental conditions were held at 240 torr and 6% CH4/H2 while 267 the substrate temperature was adjusted in the range of 950-1300 ° C by varying substrate holder designs and absorbed power. The pictures shown in each row in Figure 7.18 are the micrographs of the whole top surface and a close-up of a portion of the surface from the same sample. They were taken with 25x (whole top surface) and 100x (close-up) magnifications. Generally the samples shown in Figure 7.18 are defect free, except for the polycrystalline rims on the top surface edges of some samples. There are no rims observed in Figure 7.18(b), while some growth hillocks with unepitaxial crystallites on the edges are observed in Figure 7.18(d). This shows that the polycrystalline diamond rims formed as the diamond crystal grew thicker. The rims only occurred when the synthesized diamond reached a certain thickness (> 100 µm) and they became wider as the synthesized diamond thickness increased. In Figure 7.18(i), the sample with highest growth rate and thickest synthesized diamond film has widest polycrystalline diamond rim among all samples in this figure. The rims were measured to be approximately 0.3 mm wide. All the photos in Figure 7.18 show that the diamond grows from substrate edges inwards. Thus generally the thicknesses of synthesized diamond were thicker at edges than the thicknesses in the center. Though the rough surface of synthesized diamond could be polished off, a diamond sample with smoother surface was preferable due to less diamond sacrifice during polishing process. Thus the surface roughness was also an important 268 evaluation of diamond morphology, as well as the appearance of defects on the surface. As shown in Figure 7.18, the diamond morphology varies as substrate temperature increases and growth rate varies. For the experimental conditions of 240 torr and 6% CH4/H2, the substrate temperature range could be approximately divided into: (1) a < 1050 ° range, where the growth rate was C relatively low (< 20 µm/h), and the diamond surface was very smooth (see Figure 7.18(a)-(d)); (2) a 1050-1250 ° C range, where the growth rate increased past 20 µm/h, and the diamond surface exhibits so called macro-steps (growth steps) [98] such as the sample shown in Figure 7.18(e) or a rough surface as Figure 7.18(g) showns; and (3) a > 1250 ° range, C where the growth steps were replaced by a rough “bulky rock” like appearance (Figure 7.18(o) and (p)). When the substrate temperature was in the lower range, i.e. around 1000 ° the diamond surface was smooth. For instance the average distance C, between the highest peak and lowest valley on the samples surface as shown in Figure 7.18(a) is just around 10 µm. As substrate temperature increased, the diamond surface became rougher. There were usually two typical morphologies when substrate temperature was around 1100 ° C. As Figure 7.18(e) and (f) show, with a substrate temperature and a growth rate of 1086 ° C and 29.77 µm/h respectively, the diamond surface is covered by macro-steps [98], and the distances between peaks and valleys are between 269 50-100 µm. On the other hand, with a substrate temperature of 1140 ° and a C growth rate of 30.73 µm/h (Figure 7.18(f) and (g)), the diamond surface did not exhibit macro-steps but a rough “rocky” surface from edges inwards, and some growth hillocks were observed in the surface center. The average peak-valley distance of this sample surface is around 100 µm. Though the surface is still free of defects, the morphology with growth hillocks such as the one shown in Figure 7.18 (f) was not desirable. The morphology with growth steps was more preferable due to the relatively smoother and defect-free surface. When substrate temperature was increased further up to 1200 ° the C, diamond surface usually only displayed growth steps as shown in Figure 7.18(i)-(l). These growth steps typically aligned along the <100> directions, and were between a few micrometers (Figure 7.18(j)) and a hundred micrometers wide (Figure 7.18(l)). On the diamond surfaces there were no defects, however polycrystalline diamond rims were observed on the surface edges. These originated from and were identified as a polycrystalline diamond rim that formed as the diamond grew. In spite of the steps the whole surface was relatively smooth. In Figure 7.18(i), the peak-valley distances are below 30 µm. In this substrate temperature range, SCD growth rate also reaches a maximum in the growth rate versus substrate temperature curve as described in previous section. For these particular experimental conditions the growth rate was usually larger than 30 µm/h. When substrate temperature was higher than 1200 ° the morphology of macro-steps was gradually replaced by the C, 270 “bulky rock” like appearance which is mentioned in Section 7.4.2. As shown in Figure 7.18(o), when the substrate temperature reached 1284 ° C, the morphology of growth steps completely disappears. The sample surface is very rough and looks like big “rocks” bunching together, so called “bulky rock” like appearance. The peak-valley distances of this sample surface are more than 100 µm, which is the biggest among the roughness of the samples in Figure 7.18. This phenomenon was observed for samples with substrate temperature higher than 1250 ° C. When the substrate temperature was 1231 ° (Figure 7.18(m) and (n)), the growth steps did not cover the whole C sample surface, and the “rock” like morphology started to appear at the edges as shown in Figure 7.18(n). This shows that the diamond surface morphology gradually transformed from growth steps to rough “bulky rock” like appearance as substrate temperature increased. The micrographs in Figure 7.18 show that the diamond synthesized in the substrate temperature range for high growth rates usually exhibited the growth steps morphology, which was relatively smooth and free of defects. For lower substrate temperature, the surface might be smooth, and the associated growth rate was also lower. When substrate temperature was higher than 1250 ° the diamond surface showed the “bulky rock” appearance instead of C, growth steps, though the growth rate was still relatively high. Thus the smooth growth steps morphology and associated high growth rate was desirable for SCD synthesis. 271 Figure 7.18 – Micrographs of the deposited diamond surfaces versus substrate temperature for the samples from the experiments presented in Figure 7.13. Experimental conditions: pressure = 240 torr, CH4/H2 = 6%, L2 = 58.6 mm, Ls = 21.6 cm, Zs = -3.34 mm – -5.73 mm. Rtm is the average distances between the surface peaks and valleys. 272 Figure 7.18 (cont’d) Figure 7.19 displays the micrographs of three samples which were deposited at 240 torr with similar substrate temperatures around 1280 ° The C. sample in Figure 7.19(a) was synthesized at 1282 ° with 5% CH4/H2 and a C, growth rate of 7.09 µm/h. The surface of this sample was “bulky rock” like and the faces of the “rocks” were very smooth. As methane concentration was 273 increased to 6% and 7%, the growth rates of samples in Figure 7.19(b) and (c) increased to 25.4 µm/h and 28.34 µm/h, respectively, while their substrate temperatures were kept approximately constant. The morphologies of the samples were similar. There were no growth steps, or unepitaxial crystallites displayed on the surfaces. They all had “bulky rock” like appearance, no matter what the methane concentration was or what the growth rate was. When the growth rate was larger, the “rocks” on the surface were steeper. As described in section 7.4.2, this phenomenon also occurred when the nitrogen concentration in the gas phase was smaller than 50 ppm, otherwise the adsorbed impurities would lead to the formation of macro steps [95, 96]. Figure 7.19 – Micrographs of diamond surface of samples deposited at 240 torr with similar substrate temperatures and different methane concentrations. The rough surface of samples deposited with high substrate temperature was not preferable, thus the samples obtained from the lower substrate temperature range were explored further. Figure 7.20 displays the 274 micrographs of the diamond surface of selected samples from Figure 7.14. They were all synthesized at 240 torr with methane concentration varying from 4-7%. The micrographs in Figure 7.14(a), (b), (d) and (e) were taken from the samples which were selected from the growth rate maximum range in the growth rate versus substrate temperature curves for each methane concentration. The one with 4% CH4/H2 (Figure 7.20(a)) had a similar morphology and close growth rate to the sample deposited with 6% CH 4/H2 and low substrate temperature (Figure 7.18(c)). Both of them did not show any growth steps on the surfaces. When methane concentrations were larger than 4%, as shown in Figure 7.20(b), (d) and (e), the samples which were synthesized around their maximum growth rate for each methane concentration all exhibited the macro-steps morphology. The sample with methane concentration of 7% (Figure 7.20(e)) showed the narrowest steps and looked quite smooth. The peak-valley distances for this sample surface were around 10µm. If the substrate temperature was off the maximum growth rate range, and diamond growth rate decreased, the surface might have other morphology than the macro-steps, as shown in Figure 7.18(g) for methane concentration of 6%. Figure 7.20(c) shows an example of the surface morphology of a sample with 5% CH4/H2 and lower substrate temperature than the one in (b). Similarly, Figure 7.20 (f) shows another example of the surface morphology of a sample with 7% CH4/H2 and lower substrate temperature than the one in (e). At methane concentration of 5%, the sample 275 deposited in low substrate temperature showed no growth steps. While the sample with methane concentration of 7% and low substrate temperature of 1015 ° C exhibited the growth steps morphology. Though there were large hillocks seen on the sample surface and the growth steps did not perfectly align along the <100> directions, there were no defects observed. Figure 7.20 – Micrographs of diamond surface of samples from Figure 7.14, which were deposited at 240 torr with different methane concentrations. Observation of Figure 7.20 shows that high methane concentration was preferable from a morphological point of view. As methane concentration increased, there was a tendency to have growth steps morphology with associated high growth rate and low defects density. This result was consistent 276 with that of A. Tallaire et al. [43], which also observed the growth steps appearance with methane concentrations of 6-7%, although the growth rates obtained were lower than 15 µm/h at pressure of 165 torr. Furthermore, the influence of pressure on the diamond morphology was investigated by observing the samples presented in Figure 7.15. The surface micrographs of samples with methane concentration of 5% and pressure variation of 180-280 torr are displayed in Figure 7.21. The sample deposited at 180 torr (Figure 7.21(a)) had a similar morphology to the samples with growth rates lower than 10 µm/h displayed in figures above. When pressure was 200 torr (Figure 7.21(b)), the growth steps appeared from the edges of sample while the center part was still similar to the one at 180 torr. For SCD synthesis in the high pressure regime starting from 220 torr, the samples all exhibited the growth steps morphologies with no unepitaxial crystallites observed. The steps mostly aligned along the <100> directions as shown in Figure 7.21(c)-(f). From these observations presented above, it appears that the optimum set of input variables from the simplified multi-variable space could be found for the diamond morphology. The SCD synthesis process was preferable at pressures: (1) that were greater than 200 torr, with methane concentrations of higher than 4%, and (2) that were processed in the substrate temperature range where high growth rate were enabled but not beyond 1250 ° Under C. these experimental conditions, the diamond could be synthesized with a “growth steps” surface morphology and with no defects. 277 Figure 7.21 – Micrographs of diamond surface of samples from Figure 7.15, which were deposited with methane concentration of 5% at different pressures. 278 7.4.5 Diamond quality characterization 7.4.5.1 High quality high growth rate SCD synthesis window Except for the visual observations of the diamond surface morphologies described in the previous section, the quality of the synthesized SCD with no additional nitrogen impurities in the gas phase was also evaluated by Raman full-width-half-maximum (FWHM) and secondary ion mass spectrometry (SIMS) analysis. An example of the diamond quality versus substrate temperature within the diamond growth window is presented in Figure 7.22. The quality and associated growth rate of the synthesized diamond is plotted versus substrate temperature for a series of experiments which are selected from the data presented in Figure 7.13. As described in section 7.4.3, in these experiments the pressure was held constant at 240 torr and the methane concentration was held constant at 6%. In addition, most reactor geometrical parameters were fixed, i.e. Lp = 3.56 cm, Ls = 21.6 cm, L2 = 58.6 mm. The substrate temperature was varied in the range of 950-1300 ° by adjusting the substrate C holder designs and absorbed power. There was no extra nitrogen introduced in the gas phase in these experiments and the nitrogen impurity concentration in the gas phase was assumed to be 10 ppm or less. In Figure 7.22, the square black data points represent the diamond growth rate according to the vertical axis on the left, and the measured FWHM 279 of Raman spectra of the synthesized SCD samples are displayed as the circular red data points. The figure shows that the Raman FWHM of the -1 synthesized SCD ranged from 1.65-2.0 cm . There are two horizontal dashed lines in Figure 7.22, which display the Raman FWHM data of two reference diamond samples. The lower dashed line represents a type IIIa SCD sample -1 from Element Six with a FWHM of 1.57 cm , and the upper dashed line represents the measured Raman FWHM of 1.88 cm -1 for a typical type Ib HPHT diamond seed, which contains nitrogen impurities of 10-100 ppm. An excellent diamond quality and high growth rate substrate temperature window is observed between substrate deposition temperatures of 1030 and 1250 ° C. This is displayed in Figure 7.22 as the region between the two green dash-dot vertical dashed lines. Within this window: (1) the Raman FWHM of synthesized SCD data points are close to the reference FWHM of the Element Six type IIIa diamond sample and (2) the growth rates, are of greater than 15 µm/h. Thus within this deposition temperature window the synthesized SCD data points have a relatively smooth and defect-free surface, very good diamond quality and also have high growth rates. 280 Figure 7.22 – SCD linear growth rate and Raman FWHM versus substrate temperature for Reactor B. Experimental conditions: pressure = 240 torr, CH4/H2 = 6%, Zs = -3.34 mm – -5.73 mm. Additionally, the synthesized SCD samples shown in Figure 7.22 were measured by SIMS as well. The SIMS analysis results indicated that when the total nitrogen impurities concentration in the gas phase was controlled to be less than 10 ppm there was less than 300 ppb (below the detection limit of the SIMS measurements as described in Chapter 4) of either nitrogen or silicon in the synthesized diamond material. Figure 7.23 displays another example of diamond quality versus 281 substrate temperature within diamond growth window. The experimental data are selected from Figure 7.14 for methane concentration of 5%. The pressure was kept constant at 240 torr as well and other experimental conditions were similar to that of data presented above in Figure 7.22. Figure 7.23 – SCD linear growth rate and Raman FWHM versus substrate temperature for Reactor B. Experimental condition: pressure = 240 torr, CH4/H2 = 5%, Zs = -5.73 mm – -7.54 mm. Similar to Figure 7.22, the diamond growth rate and associated Raman FWHM are shown in Figure 7.23 as black and red data points, respectively. The Raman FWHM data of two reference SCD samples are also displayed as 282 two horizontal dashed lines. Figure 7.23 shows that the diamond quality of all samples in this figure was very good by comparing the Raman FWHM with the data of reference SCD samples. The excellent SCD quality and high growth rate window between 1030 and 1250 ° which was determined from Figure C 7.22 is also displayed in Figure 7.23 as the region between the two green dash-dot vertical dashed lines. This high quality and high growth rate window is also applicable for SCD synthesis with methane concentrations of 5% at pressure of 240 torr. By exploring the influence of substrate temperature on SCD growth rate and quality, a high quality high growth rate SCD synthesis window between 1030 and 1250 ° was identified. It is worth noting that this synthesis window C was determined for SCD synthesis at pressure of 240 torr and with methane concentration of 6%. It might also be applicable when the experimental conditions were changed a bit, such as 240 torr and methane concentration of 5%. If the pressure was changed from 240 torr to 180 torr, the synthesis window would need to be re-verified, since the safe and efficient experimental operating regime for substrate temperature is expected to vary as pressure is varied. This was described in section 7.4.3. 283 7.4.5.2 Characterization of diamond plates Since the quality of synthesized SCD samples was excellent, additional IR-UV transmission measurements and birefringence imaging were performed. These two measurement techniques both required the fabrication of diamond plates with smooth and flat surfaces which must be parallel with each other. The diamond plates were synthesized by Reactor B via a single or multistep process which was carried out within the safe and efficient reactor operating regime as identified using the methodology described in the Chapter 5. During the synthesis process the substrate temperature was controlled to be within the high quality high growth rate synthesis window as determined in the section above by adjusting the input variables such as input power, pressure, substrate position, etc. SCD diamond plates were created by laser cutting the CVD diamond layer from the diamond substrate and then mechanically polishing the final produced plate. The UV to IR transmission measurements were performed on several SCD sample plates that were synthesized for comparison in Reactors A, B, and C. Reactor A is the first reactor design, i.e. the conventional reactor which was introduced and discussed in Chapter 3 [65-67]. Like Reactor B, Reactor C was also improved from the design of Reactor A, and has similar cylindrical, phi-symmetrical geometry and identical substrate holder/cooling stage as Reactor B [79, 86, 90]. When compared to Reactor A, both Reactors B and C 284 provide reliable and safe operation at higher discharge power densities and higher pressures. The examples of the optical transmission measurements performed on the synthesized diamond plates are shown in Figure 7.24. Sample 1 was synthesized in Reactor A at 160 torr, with a 5% methane concentration, and with 11 ppm of extra nitrogen gas phase addition, while samples 2 and 3 were synthesized in Reactor C at 240 torr, 5% methane concentration and with 5 ppm and zero extra nitrogen addition respectively. Sample 4 was synthesized in Reactor B with no extra nitrogen addition at 240 torr, 6% methane concentration. The IR transmission spectra for all samples (not shown in Figure 7.24) were similar to those observed for type IIa diamond. However as is shown in Figure 7.24 the sub-band gap ultraviolet absorption coefficients are different for the four samples. For all of the samples the absorption coefficients increase with the addition of nitrogen in the gas phase. However the absorption coefficients at 250 nm for the diamond plates synthesized in -1 Reactors B and C are between 4 and 7 cm . Thus the diamond plates have transmission spectra that are similar to that of type IIa diamond [99]. 285 Figure 7.24 – Transmission measurement results. Experimental conditions: 160 torr, CH4/H2 = 5% (Reactor A); 240 torr, CH4/H2 = 6% (Reactor B); 240 torr, CH4/H2 = 5% (Reactor C). The SCD plates synthesized by Reactor B were also evaluated by birefringence imaging. By using a set of polarization filters as is described in Chapter 4, birefringence imaging reveals the internal stress patterns in the crystal. Examples of micrographs from birefringence imaging are shown in Figure 7.25. The sample displayed in Figure 7.25(a) and (c) was synthesized in Reactor A at 160 torr, 5% methane concentration, and with 22 ppm extra nitrogen addition in the gas phase. The other sample (Figure 7.25(b) and (d)) 286 was synthesized in Reactor B at 240 torr, 6% methane concentration, with no extra nitrogen addition, substrate temperature of 1079 ° and with growth rate C of 23.76 µm/h. The two samples from Reactor A and B were both free-standing SCD plates, with thicknesses of 1.5 mm and 0.5 mm respectively. The two micrographs in Figure 7.25(a) and (b) were taken without birefringence imaging set, while the birefringence micrographs in Figure 7.25(c) and (d) were taken with exposure time of 100 ms. As shown in Figure 7.25(c), the sample from Reactor A shows several intensity contrast lines in the bottom area which might indicate the diamond growth mode forming steps [100]. Other regions of this sample display random patterns, which reveal the complicated stress distribution in this sample. The birefringence micrograph of the sample from Reactor B (Figure 7.25(d)) has a relatively large, dark and uniform area in the center, which shows that this sample was almost entirely in an impressively low internal stress level. Compared with the top surface picture of this sample as shown in Figure 7.25(b), there are intensity contrast lines occurring near the edges of sample which indicate the defects which might originate from cutting and polishing process or due to the presence of a small polycrystalline rim. The sample from Reactor B also exhibited high stress level at the edges, which might be caused by the polycrystalline diamond rims along the crystal edges that formed during the synthesis process. It was possible that the polycrystalline diamond rims extended into the synthesized SCD layer then reduced the crystal quality and 287 increased the internal stresses. Note that the edges of this sample were not trimmed and polished. This is shown in Figure 7.25(b). Figure 7.25 – Examples of micrographs with light and from birefringence imaging for Reactors A and B. As presented above, the quality of synthesized SCD in Reactor B was assessed by many methods. The Raman FWHM ranging from 1.65-1.80 cm -1 was obtained from the samples synthesized in the high quality high growth rate 288 window. SIMS analysis results indicated that there was less than 300 ppb nitrogen existing in synthesized diamond when total nitrogen concentration was less than 10 ppm in the gas phase, i.e. the nitrogen content in the crystal was below the SIMS detection limit. The IR-UV transmission spectra of the diamond from Reactor B indicated that the synthesized diamond was similar to type IIa diamond. Specifically, the sub-band gap ultraviolet optical absorption coefficients for the diamond from Reactor B was comparable to that reported for type IIa diamond. In addition, the birefringence imaging showed that a sample from Reactor B had relatively low internal stress level and uniform stress distribution. In conclusion, Reactor B was capable of synthesizing SCD of excellent quality (type IIa: gem quality or better) with high growth rates within the growth window of 1030-1250 ° at high pressures. C 7.4.6 Summary In order to identify a safe, efficient, and robust experimental process window within the high pressure regime for production of high quality high growth rate SCD, the influence of important experimental input variables such as substrate position, gas chemistry, substrate temperature, and pressure etc. on the diamond synthesis process was studied by performing SCD synthesis experiments over a range of experimental conditions. The synthesized diamond was evaluated by growth rate, substrate surface morphology, Raman 289 spectroscopy, SIMS analysis, IR-UV transmission, and birefringence imaging measurement techniques. The role of each single experimental variable in SCD deposition process was explored by analyzing the experimental results. The experimental variables of the safe and efficient reactor operating window for high quality high growth rate SCD synthesis are summarized in Table 7.5. By evaluating Reactor B’s performance in the SCD synthesis application at high pressures, and taking safety into account, the length of the coaxial section of the reactor was chosen at 58.6 mm, and Ls was also fixed at 21.6 cm, as shown in Table 7.5. Thus Reactor B could be operated safely and robustly at high pressures from 180-280 torr and these adjustments enabled high growth rate SCD synthesis. Linear growth rates of 40-50 µm/h were possible when total nitrogen impurity in the gas phase was less than 10 ppm. Contrary to the claim that the acceptable temperature range for SCD synthesis was narrow and there was only little room for growth rate improvement by adjusting substrate temperature [57], at a constant pressure of 240 a broad SCD growth window was observed between 950 and 1300 ° for Reactor B C and the magnitude of the growth rate could be varied by a factor of 3 within this window. The SCD synthesis experiments demonstrated that SCD growth rates increased as pressure, methane concentration, nitrogen concentration, and discharge power density increased. As pressure was increased the high quality experimental substrate temperature window also expanded and moved 290 along substrate temperature space. The larger methane concentrations (> 4%) can synthesize high quality diamond. The upper limit of methane concentrations has not been determined yet. Since the operation with methane concentration of 9% was reported to cause soot problems [86], the largest methane concentration that used in this dissertation research was chosen to be 7% to avoid soot formation. The observation of synthesized diamond morphology demonstrated that the samples synthesized within high growth rate substrate temperature window at higher pressures (> 200 torr) with larger methane concentrations (> 4%) synthesized diamond with a smooth surface with growth steps morphology and with no observed unepitaxial defects. When nitrogen impurity levels in the gas phase were reduced below 10 ppm the quality of the synthesized diamond was assessed as type IIa or better. At an operating pressure of 240 torr a high quality high growth rate SCD synthesis window was experimentally identified between 1030 and 1250 ° C. After laser cutting and polishing high quality diamond plates were fabricated. The birefringence imaging results showed that the diamond plates had relatively low internal stress level and uniform stress distribution. These experiments results support the hypothesis that MPACVD diamond synthesis rates and diamond quality increase and improve respectively as the operating pressure increases. The determination of safe, efficient and robust reactor operating regime for high quality high growth rate SCD synthesis enables Reactor B to produce a stable discharge for a long 291 time (one to many days) and to synthesis SCD with larger thickness. Table 7.5 – Experimental variables for high quality high growth rate SCD synthesis process. Reactor tuning Probe position: Lp 3.56 cm Short position: Ls 21.6 cm Cooling stage position: L2 58.6 mm Substrate position: Zs -3 mm - -6 mm Pressure > 200 torr Absorbed power 1.6-2.0 kW Gas chemistry Methane concentration > 4% CH4/H2 Nitrogen concentration 0 extra addition Substrate temperature 1030-1250 ° C Expected growth rate 20-50 µm/h 292 7.5 Synthesis of large area single crystal diamond plates 7.5.1 Introduction Among the wide bandgap materials, diamond has the best properties for power electronic devices and has the potential to be the next wide bandgap material exploited. Currently one of the bottlenecks, which limit the electronic applications of diamond, is the lack of high quality, low cost SCD plates that have a sufficient area for electronic device fabrication. In addition to the high quality high growth rate SCD synthesis that was enabled by using Reactor B as described in Section 7.4 above, the synthesis of large area diamond plates was also briefly explored. This section introduces the concept of increasing the size of SCD substrates, the associated experimental methodology that would enable the fabrication of large MPACVD synthesized diamond plates, and the problems related to the development of the process. The initial results of some exploratory experiments directed toward the fabrication of large size SCD plates are also presented. 7.5.2 Approach of large size single crystal diamond plates synthesis The fabrication process of large size SCD plates is similar to the processes described in Section 7.4.5 for the 3.5 mm x 3.5 mm size SCD plate production. It consists of MPACVD diamond synthesis, laser cutting the grown 293 material from the seed substrate and then polishing. Since the size of synthesized SCD depends on that of the substrate seed, the synthesis of large size SCD plates requires SCD seed substrates with a large area. The biggest area of commercially available HPHT SCD substrates is approximately 6 mm x 6 mm. Thus in order to routinely fabricate large size SCD plates, it is necessary to firstly synthesize a SCD seed substrate by MPACVD with an area larger than 6 mm x 6 mm. There are a couple of methods that have the potential for increasing the area of the SCD seed substrate. Y. Mokuno et al. used a “side-surface growth method” to synthesize SCD plates with areas larger than 10 mm x 10 mm [32]. Recently H. Yamada et al. [36, 37] described a so-called “mosaic wafers” method that enabled the fabrication of 1 inch square size diamond wafers. However the “mosaic wafer” method has to be carried out by using the lift-off process with ion implantation. This method is unavailable in our research laboratories. Thus in this dissertation the side-surface growth method was employed to investigate how to increase the area of the seed SCD substrate. Figure 7.26 shows the steps of large area SCD substrates synthesis process by the side-surface growth method. As shown in Figure 7.26(a), the fabrication of large size SCD substrates started with a large commercially available HPHT diamond seed, i.e. a seed with an area of 6 mm x 6 mm. The sides of this seed were used as the first deposition surfaces. MPACVD diamond with thicknesses of a few millimeters was deposited on these 294 surfaces. See for example Figure 7.26(b). This produced an expanded substrate area of around 10 mm x 10 mm. The small arrows as shown in Figure 7.26(b) indicate the diamond growth directions. Then on the top surface of the expanded substrate, MPACVD diamond was deposited with a thickness of several millimeters or more (Figure 7.26(c)). This newly deposited SCD layer would be removed from the expanded substrate and sliced into several new SCD substrates with area of 10 mm x 10 mm (Figure 7.26(d)). Thus the SCD substrates were fabricated with area increased from 6 mm x 6 mm to 10 mm x 10 mm. This procedure can be repeated to synthesize even larger SCD substrates. One of the major challenges in this large area SCD substrate fabrication process is the single or multi-step SCD synthesis with at least a thickness of 2 mm. For single step synthesis, it is required to provide a very stable and robust discharge for at least a couple of days assuming a growth rate of 30-40 µm/h. It is also a challenge to design an appropriate substrate holder which is able to maintain the substrate temperature within the high quality high growth rate window despite the variation of diamond thickness during the process. On the other hand, if the synthesis process was divided into several steps, the sample would be taken out for cleaning and changing the substrate holder between each other. Then it is important to ensure the quality of diamond after the interruption of each experimental run. Since the key point of the side-surface growth method is to use the side of a normal HPHT seed as growth surface, it 295 is also necessary to demonstrate the SCD synthesis on the side of HPHT seed. Note that in the large area SCD substrates synthesis procedure, the substrate holder has to be varied as the deposition surface was changed due to the variation of the substrate size in the process. Figure 7.26 – Process steps of large area SCD substrates synthesis by the side-surface growth method. The small arrows indicate the diamond growth directions. 296 7.5.3 Initial results of large area single crystal diamond substrate synthesis Since the large area SCD substrate synthesis process is still under development in our laboratories, this section only presents the initial results of some first exploratory experimental runs. These first experiments focused on the demonstration of SCD synthesis on sides of a HPHT seed, involving the SCD synthesis runs of over 10 hours, and then the resumption of the synthesis process after interruption. 7.5.3.1 Single crystal diamond synthesis on sides of HPHT diamond seed In order to make use of current substrate holders, the normal HPHT diamond seeds (3.5 mm x 3.5 mm x 1.4 mm) were laser cut in half into smaller diamond substrates with sizes of around 3.5 mm x 1.7 mm x 1.4 mm. The smaller diamond was then flipped over on its side and was then used as a 3.5 mm x 1.4 mm x 1.7 mm seed. The side of original seed turned into the new top surface with area of 3.5 mm x 1.4 mm. Then this surface was polished and was prepared to be a deposition surface. According to the experimental variables summarized for high quality high growth rate SCD synthesis in Section 7.4.6, the experimental conditions for the exploratory side-surface SCD synthesis experiments were set at 240 297 torr, 7% CH4/H2, 0 N2/ H2, Zs = -3.34 mm. A growth rate of 30-40 µm/h was expected as substrate temperature was controlled in the range of 1050-1100 ° C. Figure 7.27 shows an example of the micrographs of a sample after 8-hour side-surface growth. In this experiment, the substrate temperature was 1081 ° and a growth rate of 31.83 µm/h was obtained. As shown in Figure C, 7.27(a), the deposition surface displays the similar morphology of growth steps to the samples synthesized within the same growth window presented in section 7.4.4. Though there are polycrystalline diamond rims on the edges of sample, the most of the area in the center is defect free and flat. This can be seen in more detail in the close-up view in Figure 7.27(b) with magnification of 100x. Figure 7.27 – Example of micrographs of a sample after side-surface synthesis. Experimental conditions: pressure = 240 torr, CH4/H2 = 7%, Zs = -3.21 mm. (a) 25x magnification, (b) 100x magnification. 298 The results of exploratory side-surface SCD synthesis experiments show that there was no or little difference between the sides and top surfaces of a normal HPHT diamond seed in MPACVD SCD synthesis. The sides can be considered as deposition surfaces as well. The entire reactor and processes details that were determined in the investigation of SCD synthesis in Chapters 3-7 are applicable to the side-surface synthesis. The identification of high quality high growth rate synthesis window and the process controlling via adjustment of input variables can be employed in the large area SCD substrates synthesis process as well. 7.5.3.2 Long-time single crystal diamond synthesis process In order to increase the size of SCD plates/substrates, the development of a process of long-time SCD synthesis is required. Since most of the SCD synthesis experiments investigated and reported in this dissertation were performed in than less than 10 hours, it was also necessary to explore the variation of experimental conditions versus time for very long experimental run times. Specifically the substrate temperature has to be maintained within the desired growth rate window during the entire run in order to achieve deposition with both high quality and high growth rate. A series of SCD synthesis experiments with deposition time longer than 10 hours was performed. According to the experimental variables summarized 299 for high quality high growth rate SCD synthesis in section 7.4.6, the experiment conditions for the exploratory long-time synthesis experiments was set at 240 torr, 6-7% CH4/H2, 0 N2/ H2, Zs = -3.34 mm. The range of substrate temperature was chosen between 1050 and 1100 ° which is in the lower C, range of the curve of the dependency of diamond growth rate and substrate temperature. Since the diamond thickness increases during the deposition process, and the deposition surface gets closer to discharge, then unless something is varied during the process the substrate temperature increases versus time. It was expected that there would be enough room for adjustment of the substrate temperature within the growth rate window if it was set in the lower temperature range. A growth rate of 20-40 µm/h was estimated if the substrate temperature was controlled in this range as planned. The substrate temperature controlling process versus time is described in Chapter 6. Figure 7.28 shows a successful example of SCD synthesis for 24 hours. The experiment was performed at 240 torr with methane concentration of 6%. The substrate temperature was 1079 ° and growth rate was 23.76 µm/h. C, Thus the diamond film with a thickness of 570 µm was obtained. As Figure 7.28 shows, the sample exhibited a flat and smooth surface with the typical “growth steps morphology” as described in Section 7.4.4. 300 Figure 7.28 – Example of micrographs of a sample after 24-hour deposition. Experimental condition: pressure = 240 torr, CH4/H2 = 6%, Zs = -3.34 mm. (a) 25x magnification, (b) 100x magnification. The results of these long-time SCD synthesis experiments demonstrate the possibility of the synthesis of high quality and thick diamond films. However, there were several experiments failing during the synthesis process because the quartz bell jar was burned after running for 20 hours. Thus it is important to keep the substrate temperature within a lower range where lower input power is required and the discharge is located further away from the bell jar wall. If this safety issue is resolved then further exploration can be performed for long time synthesis. High quality free standing SCD plates with thickness of more than 1 mm in a single run are expected. 301 7.5.3.3 Synthesis resumption after interruption It often is necessary to change substrate holder designs during a multistep experimental run. Each experimental run has a CVD synthesized thickness target of approximately of a few millimeters. Thus it is necessary to maintain the morphology and quality of diamond from run to run. Between two separate experiments, the sample, the substrate holder, the cooling stage and the whole chamber must be cleaned thoroughly as described in Chapter 4. However, there is still possibility of introducing impurities or defects into the crystal due to the run interruption. A series of resumption synthesis experiments were carried out to explore this problem. Figure 7.29 show an example of the experimental results. The sample was deposited for three times at 240 torr with a methane concentration of 7%. The deposition time for the first two runs and the third run are 8 hours and 10 hours, respectively. The substrate position was varied within 2 mm since the substrate holder was changed from run to run. It was ensured that the top surface of substrate was always lower than the top surface of the substrate holder when the experimental run started. As shown in Figure 7.29(a) and (b), the sample exhibits smooth and defect free surface with growth steps morphology, which indicates the first synthesis resumption was successful. However, after the third run, the sample surface was very rough, displaying many unepitaxial crystallites, which are accompanied by a relatively low growth rate, as shown in Figure 7.29(c). 302 Figure 7.29 – Micrographs of an example sample after 3 resumption synthesis experiments. Experimental conditions: pressure = 240 torr, CH4/H2 = 7%, (a) Zs = -4.49 mm, (b) Zs = -3.34 mm, (c) Zs = -3.21 mm. The results of the resumption synthesis experiments show that it may be difficult to resume the synthesis twice without deteriorating the synthesized diamond quality under current experimental conditions. However, the first resumption synthesis usually leads to excellent results. In order to increase the number of acceptable resumption synthesis, further investigation of experimental variables, such as substrate holder design and gas chemistry is required. Currently, according to the initial results presented above, a high quality SCD film with thickness of over 1 mm could be synthesized successfully by operating the synthesis experiment for 24 hours twice. Thus, it is expected that with a series of appropriate substrate holders, a SCD substrate with area of 8 mm x 8 mm could be fabricated from a 6 mm x 6 mm SCD seed. This still requires further experimental verification. 303 CHAPTER 8 SUMMARY AND RECOMMENTDATION 8.1 Summary In order to achieve safe, efficient, and stable operation in the higher pressure regime (180-280 torr) a new reactor operational principle of microwave plasma assisted chemical vapor deposition (MPACVD) reactor was demonstrated for Reactor B. This principle was to operate the reactor so that the discharge was always located away from the reactor walls and to position it next to and in good contact with the substrate. The implementation of this operational principle controlled the position of the discharge, restricted the discharge location away from the reactor walls, enabled good discharge contact with the substrate, and controlled the size of the discharge. When operating under these conditions, discharge power density was able to be increased by increasing operating pressure, reactor wall reactions were minimized, and the safe, efficient and low maintenance operation of Reactor B was established over a wide range of operating conditions. An experimental methodology was presented that determined this efficient and safe operating diamond synthesis regime for Reactor B. This methodology first experimentally defined the nonlinear relationships between the input power, discharge average power density, pressure and substrate 304 temperature; i.e. it established the operating field map at high pressure and power densities for Reactor B. Then the safe and efficient reactor operating variable space over the 180-280 torr pressure regime was defined within this operating field map. At each operating pressure the operating field map: (1) restricted and defined the upper and lower input power limits of the reactor, (2) determined the discharge absorbed power density behavior versus input power, and given a particular substrate holder configuration, (3) established the relationship between substrate temperature and absorbed power density versus substrate holder position. The experimental discharge and operating field map data are also invaluable experimental data that provide an improved understanding of the reactor performance at high pressure. The operating field map measurements along with the associated discharge photographs are also expected to be useful benchmark experimental data for evaluating numerical reactor models. More specific details of the Reactor B field map and the associated experimental methodology are summarized in Section 5.3. For Reactor B the operating field map and the safe and efficient operating regime was specifically defined for single crystal diamond (SCD) synthesis. While operating the reactor within a safe and efficient operating window, SCD synthesis was then demonstrated from 180 to 280 torr. At a constant pressure of 240 torr a high quality, high growth rate substrate temperature window was experimentally identified between 1030 and 1250 ° C. In particular using feed gases with nitrogen impurity levels of less than 10 ppm 305 SCD was synthesized with growth rates of 20-45 µm/h. The detailed SCD synthesis process recipes are summarized in Section 7.4.6 of this dissertation. The SCD synthesis experiments demonstrated that as the pressure and discharge absorbed power density increased the diamond deposition rate increased. Diamond synthesis rates and quality surpassed those that were achieved by synthesizing SCD at lower pressure and with earlier reactor technologies. As pressure was increased the experimental variable window to grow high quality diamond also expanded and larger methane concentrations (5-7%) synthesized high quality diamond. The Raman FWHM ranging from 1.65-1.80 cm -1 was obtained from the samples synthesized in the high quality, high growth rate window when nitrogen impurity levels were reduced below 10 ppm in the gas phase. SIMS analysis results indicated that there was less than 300 ppb nitrogen existing in the synthesized diamond, i.e. the nitrogen content in the crystal was below the SIMS detection limit. The IR-UV transmission spectra of the diamond from Reactor B indicated that the synthesized diamond was similar to type IIa diamond. Specifically, the sub-band gap ultraviolet optical absorption coefficients for the diamond from Reactor B was comparable to that reported for type IIa diamond. After laser cutting and polishing high quality diamond blocks and plates were synthesized. The initial birefringence imaging characterization results showed that the diamond plates had relatively low internal stress level and uniform stress distribution. These experiments supported the hypothesis that MPACVD diamond synthesis rates 306 and diamond quality increased and improved respectively as the operating pressure increased. Reactor B demonstrated robust SCD deposition at high pressures and power densities. By a combination of process control via reactor input adjustment and improved reactor design, SCD was synthesized robustly both in the short term (a few hours to a few days) by producing a very stable discharge in good contact with the substrate and in the long term (one to several years) over many hundreds of separate experiments with little reactor maintenance. Figure 8.1 summarizes the linear polycrystalline growth rates versus discharge area power density for different CVD diamond synthesis reactor designs/processes that were produced in commercialized reactors before 1997 [76]. Note that the current SCD CVD diamond synthesis growth rates that were reported recently by other groups and are summarized in Chapter 2 are located within the gray cross-hatched area. The current MPACVD diamond synthesis from our previous work [78, 79, 90] and the results from this dissertation are also placed in the grey region as indicated by the arrow shown in Figure 8.1. Certainly the diamond synthesis quality and growth rates have improved considerably since 1997. 307 Figure 8.1 – Linear growth rate versus area power density for different CVD diamond deposition reactors [76]. 1. HFCVD; 2. conventional DC CVD; 3. enclosed DC Arcjet CVD; 4. Atmosphere DC Arcjet CVD; 7. RF thermal plasma CVD; 8. magneto-microwave CVD; 9. Tubular microwave CVD; 10. Microwave plasma jet CVD; 11. ASTeX bell jar microwave CVD; 12. MSU bell jar microwave CVD; 13. MSU bell jar microwave CVD (high pressure). The gray crosshatched area indicates the location where the SCD synthesis results from this dissertation research are located on the figure. In particular the results from this dissertation indicated that with a input power of around 2 kW and nitrogen impurity levels of less than 10 ppm type IIa 308 SCD was synthesized with growth rates of 10-45 µm/h (see Figures 7.14 and 7.15) in the 180-280 torr pressure regime, while still achieving excellent electric energy synthesis efficiencies of 4.5-20 kW-h/ct over a one inch diameter deposition surface. 8.2 Publications resulting from this dissertation research The detailed experimental SCD synthesis behavior described in this dissertation for Reactor B while operating within the high pressure regime has been published [79, 90]. The experimental methodology that defines the safe and efficient experimental operational regime and the determination of the operating field map for Reactor B in the high pressure regime were described. The influence of input experimental variables on MPACVD SCD synthesis and the identification of the high quality high growth rate SCD synthesis window were also presented. A patent concerned with the reactor design of Reactor B has been issued [78]. 8.3 A Comparison of the experimental data from this dissertation with the recent work of others As summarized in Chapter 2, the MPACVD SCD synthesis at high 309 pressures was carried out and investigated recently by other research groups as well. There are some similarities and differences between the SCD experimental results presented in this dissertation and the results of other research groups. For example, Y. Mokuno et al. reported a recipe for high growth rate SCD synthesis [32] which indicated the detailed experimental conditions such as gas chemistry and substrate temperature range. The growth rates of 30-50 µm/h were obtained at pressures of 160-180 torr. Though the growth rates were comparable with the experimental results from this dissertation research, they needed much more power (around 3 kW versus our 2 kW or less), and also added considerably more N to the gas phase (1200 ppm N2/H2) in order to achieve similar results. In the pressure range of 100-200 torr, A. Tallaire et al. reported that the addition of nitrogen even at rates as low as a few ppm in the gas phase was enough to increase the growth rate [45, 46, 57]. They also reported the increase of growth rate versus high power densities [57, 97] and the variation of growth rate versus substrate temperature at a constant nitrogen concentration [46]. Their results showed similar trends for the variation of growth rate versus several experimental variables as were observed in the results from this dissertation research. However, due to the differences of the reactor designs, the method of reactor matching and the method of measurement of the substrate temperature, the absorbed power densities, etc. the two results cannot be easily compared. Additionally the size of their 310 experimental data set concerned with the growth rate versus basic experimental variables such as substrate temperature was not as large as the experimental data set generated for this dissertation research activity. 8.4 Recommendations for future research Based on the current results presented in this dissertation, there are a number of recommendations that should be investigated further. Listed below are examples of future or follow up work for MPACVD SCD synthesis.  Further investigate the methodology of defining the safe and efficient experimental operational regime for Reactor B. Specifically the method of determining the discharge volume could be improved by using a specific-wavelength filter in photography, and then by comparing the results, i.e. the magnitude and spatial variation of the species densities with the results from numerical modeling investigations of the discharge.  Develop a fabrication process for large area SCD plate synthesis. This includes large area SCD substrate synthesis and deposition of SCD layers with several millimeter thicknesses on the large area substrate.  Improve the diamond quality by reducing the impurities levels in the gas phase. This could be achieved by employing feeding gases with higher 311 purities and by adding turbo pump to the experimental system. It would be worth investigating the variation of growth rate in the experimental environment with fewer impurities as well.  Investigate the SCD synthesis with pulsed input power. This with the same average input microwave power has the potential to increase the SCD growth rates compared with the synthesis with continuous mode input power without sacrificing the diamond quality.  Measure the species presence in the gas phase by emission spectroscopy. This would help check the impurities levels and further analyze the SCD synthesis at high pressures.  Investigate MPACVD diamond synthesis at even higher pressures, such as 280-500 torr. The discharge power density and species densities would increase further, which might increase the diamond growth rate and improve the diamond quality further.  Explore diamond synthesis at higher methane concentrations of larger than 7%. This would increase diamond growth rate, but might cause soot formation during the deposition process. 312 APPENDICES 313 APPENDIX A Configurations of Substrate Holders Table A.1 – Configurations of substrate holders for single crystal diamond synthesis experiments. The schematic drawings of holders are shown in Figures 3.17-3.20 and A.1. Holder Additional Holder Pocket Pocket Pocket Pocket L1 (mm) insert thickness inner outer inner outer (mm) (mm) depth depth width width (mm) (mm) (mm) (mm) 1 N/A 1.37 0.60 1.79 4 7 52.66 2 N/A 2.24 N/A 0.99 N/A 7 52.87 3 N/A 2.94 N/A 0.56 N/A 4 53.00 4 N/A 2.64 1.12 2.67 4 7 54.81 4+ 0.58 mm 3.22 1.12 2.67 4 7 55.39 5 N/A 3.37 0.85 1.81 4 6 54.68 5+ 0.58 mm 3.95 0.85 1.81 4 6 55.26 6 N/A 2.61 0.60 1.40 4 5 53.53 6+ 0.58 mm 3.19 0.60 1.40 4 5 54.11 314 Figure A.1 – Schematic drawing of a pocket substrate holder for single crystal diamond synthesis experiments. The dimensions summarized in Table A.1 are indicated in this figure. 315 APPENDIX B Calculation of Nitrogen Impurity Concentration For experiments performed without extra nitrogen addition and with a vacuum system leak rate of 1 mtorr/h, the input gas impurities were calculated as below.  For nitrogen impurity from vacuum system leak The system volume was estimated at 78200 cm 3 from the measurement for system dimensions. With a vacuum system leak rate of 1 mtorr/h, the flow rate of atmosphere from leak is calculated as (1 x 10 -3 3 torr / 60 min) x (78200 cm / 760 torr) = 1.715 x 10 -3 sccm. With a constant flow rate of hydrogen in the experimental runs of 400 sccm and a nitrogen concentration in the atmosphere of 78%, the nitrogen impurity level from system leak is calculated as 1.715 x 10  -3 sccm / 400 sccm = 3.344 ppm. For nitrogen impurity from feed gases The purity grades of feed gases are 5-5N for hydrogen and 5N for methane. When all impurities in the feed gases were assumed as nitrogen, with a constant flow rate of hydrogen of 400 sccm and a methane concentration of 5% for example, the nitrogen impurity level from feed gases is calculated as 316 1 – 99.9995% + (1 – 99.999%) x 5% = 5.5 ppm. Thus, without extra nitrogen addition and with a vacuum system leak rate of 1 mtorr/h, the nitrogen impurity concentration in the gas phase was calculated to be less than 10 ppm. 317 APPENDIX C Experimental Data for Diamond Etching Table C.1 – Fixed experimental variables used in the hydrogen only plasma etching experiments. Length of short (Ls) 21.6 cm Length of probe (Lp) 3.56 cm Cooling stage position (L2) 58.6 mm Gas chemistry: H2 400 sccm (purity grade: 99.9995%) Pressure (p) 240 torr Operation time 1h Table C.2 – Experimental data for single crystal diamond etching experiments. Sample Surface Holder Zs Etching Etching (kW) temperature rate (° C) (mm) Pabs (µm/h) 39 Top 3 -5.6 1.40 1467 6.78 39 Bottom 2 -5.73 1.70 1200 0.83 40 Bottom 2 -5.73 1.65 1035 3.40 40 Top 2 -5.73 1.80 1093 0.30 41 Top 3 -5.6 1.43 1567 10.26 318 Table C.2 (cont’d) 41 Bottom 3 -5.6 1.45 1577 12.93 42 Top 4 -3.79 2.20 834 2.13 43 Bottom 3 -5.6 1.40 1441 9.37 43 Top 3 -5.6 1.42 1497 6.28 43 Top 3 -5.6 1.42 1500 9.15 44 Top 3 -5.6 1.42 1477 7.70 44 Bottom 3 -5.6 1.50 1517 8.47 45 Top 3 -5.6 1.40 1577 7.44 45 Bottom 3 -5.6 1.42 1546 6.23 51 Bottom 3 -5.6 1.52 1487 8.27 51 Top 3 -5.6 1.40 1522 10.05 52 Bottom 3 -5.6 1.45 1477 7.46 53 Top 3 -5.6 1.45 1527 6.46 54 Bottom 3 -5.6 1.45 1554 9.82 55 Bottom 3 -5.6 1.40 1475 6.85 56 Bottom 3 -5.6 1.40 1506 8.25 319 APPENDIX D Experimental Data for Diamond Synthesis Table D.1 – Fixed experimental variables for the single crystal diamond synthesis experiments. Fixed input experimental variables Reactor design Reactor B Substrates Single crystal diamond Length of probe (Lp) 3.56 cm Gas chemistry: H2 400 sccm (purity grade: 99.9995%) 320 Table D.2 – The flow rates of methane employed in single crystal diamond synthesis experiments for various concentrations. CH4/H2 4% 5% 6% 7% CH4 flow rates (sccm) 16 20 24 28 Table D.3 – The flow rates of nitrogen employed in single crystal diamond synthesis experiments for various concentrations. N2/H2 (ppm) 25 50 100 150 200 N2 flow rates (sccm) 1 2 4 6 8 Table D.4 – The associated positions of short plate (Ls) for varied L2 employed in single crystal diamond synthesis experiments. L2 (mm) 52.1 55.33 58.6 60.2 61.9 Ls (cm) 21.89 21.79 21.6 21.46 21.46 321 Table D.5 – Experimental data for single crystal diamond synthesis experiments. The information about other variables such as L1, Ls and feed gases flow rates can be looked up in Tables A.1, and D.1-D.4. Sample Pressure CH4/H2 N2/H2 (torr) Holder L2 (mm) (ppm) Growth Pabs time (h) Zs (mm) Ts (° C) (kW) Thickness Growth (µm) rate (µm/h) 3 240 5% 0 3 58.6 -5.6 8 2.03 1145 223.14 27.89 KWH109 240 5% 0 1 60.2 -7.54 8 2.27 1148 143.96 18.00 5 220 5% 0 1 60.2 -7.54 8 2.03 1073 71.93 8.99 7 220 5% 50 1 60.2 -7.54 8 2.06 1081 145.16 18.14 8 240 5% 100 1 60.2 -7.54 8 2.20 1172 375.93 46.99 10 240 5% 50 1 60.2 -7.54 8 2.00 1120 258.92 32.36 11 240 5% 100 1 60.2 -7.54 8 2.10 1149 356.27 44.53 322 Table D.5 (cont’d) 19 220 5% 50 1 60.2 -7.54 8 2.18 1117 193.41 24.18 24 240 5% 0 1 60.2 -7.54 23 2.08 1076 233.69 10.16 25 220 5% 0 1 60.2 -7.54 21 2.13 1062 217.64 10.36 31 240 5% 0 3 58.6 -5.6 10 1.40 1361 78.33 7.83 32 240 5% 0 2 58.6 -5.73 10 1.67 1282 70.85 7.09 34 240 5% 25 2 58.6 -5.73 10 2.00 1234 98.57 9.86 35 240 5% 50 2 58.6 -5.73 8 2.00 1307 157.72 19.71 36 240 5% 150 2 58.6 -5.73 8 1.98 1318 325.97 40.75 38 240 5% 200 2 58.6 -5.73 8 1.98 1326 361.62 45.20 th 240 5% 0 2 58.6 -5.73 10 2.20 1070 110.41 11.04 th 240 5% 0 2 58.6 -5.73 10 1.89 1201 252.50 25.25 39 (5 ) 40 (4 ) 323 Table D.5 (cont’d) 43 240 5% 0 3 58.6 -5.6 8 1.41 1247 34.53 4.32 44 240 5% 0 3 58.6 -5.6 10 1.42 1242 113.18 11.32 49 240 6% 0 3 58.6 -5.6 5 1.40 1276 50.14 10.03 60 240 6% 0 6 58.6 -5.07 8 1.92 1043 131.28 16.41 61 240 5% 0 6 58.6 -5.07 8 1.94 1024 99.90 12.49 63 240 5% 0 1 60.2 -7.54 8 2.06 994 74.51 9.31 64 240 5% 0 1 61.9 -9.24 8 2.10 996 87.29 10.91 65 240 5% 0 1 58.6 -5.94 8 2.00 984 65.76 8.22 66 240 5% 0 1 55.33 -2.67 8 1.99 989 48.17 6.02 67 240 5% 0 1 52.1 0.56 8 1.84 935 27.31 3.41 69 240 6% 0 6 58.6 -5.07 8 1.86 1030 122.05 15.26 324 Table D.5 (cont’d) 70 240 6% 0 6+ 58.6 -4.49 8 1.72 1081 226.74 28.34 70 (2 ) 240 6% 0 5 58.6 -3.92 8 1.75 1096 238.14 29.77 71 240 6% 0 6 58.6 -5.07 8 1.69 1140 245.81 30.73 72 240 6% 0 6 58.6 -5.07 8 1.90 1057 114.32 14.29 73 240 6% 0 6 58.6 -5.07 8 1.84 1036 144.27 18.03 74 240 6% 0 6 58.6 -5.07 8 1.70 978 107.86 13.48 75 240 6% 0 3 58.6 -5.6 8 1.40 1284 203.18 25.40 76 240 6% 0 3 58.6 -5.6 8 1.42 1252 176.68 22.09 77 240 6% 0 6 58.6 -5.07 7.5 1.80 1170 307.56 41.01 78 240 6% 0 6 58.6 -5.07 8 1.90 1162 316.53 39.57 79 240 6% 0 6+ 58.6 -4.49 8 1.80 1200 274.30 34.29 nd 325 Table D.5 (cont’d) 80 240 6% 0 6+ 58.6 -4.49 8 2.10 1231 186.71 23.34 82 240 4% 0 3 58.6 -5.6 8 1.45 1268 41.51 5.19 85 240 4% 0 3 58.6 -5.6 8 1.40 1226 25.42 3.18 86 240 4% 0 6 58.6 -5.07 8 1.91 1135 133.35 16.67 87 240 4% 0 6+ 58.6 -4.49 8 1.92 1171 62.76 7.84 89 240 4% 0 6+ 58.6 -4.49 8 1.92 1093 61.22 7.65 91 240 4% 0 6+ 58.6 -4.49 8 1.70 1052 67.69 8.46 93 240 4% 0 6 58.6 -5.07 8 1.95 986 76.01 9.50 96 240 5% 0 6+ 58.6 -4.49 8 1.47 1104 209.19 26.15 97 240 5% 0 6 58.6 -5.07 8 1.99 1017 69.19 8.65 98 240 5% 0 6+ 58.6 -4.49 8 1.54 1071 177.20 22.15 326 Table D.5 (cont’d) 102 240 6% 0 5+ 58.6 -3.34 8 1.76 1063 208.96 26.12 103 240 6% 0 5+ 58.6 -3.34 24 1.74 1079 570.25 23.76 111 240 6% 0 5+ 58.6 -3.34 8 1.76 1057 136.05 17.01 112 240 6% 0 6+ 58.6 -4.49 8 1.73 1057 132.04 16.50 113 240 6% 0 3 58.6 -5.6 8 1.40 1225 171.83 21.48 114 240 6% 0 3 58.6 -5.6 8 1.50 1276 150.60 18.82 115 240 7% 0 3 58.6 -5.6 8 1.59 1281 226.75 28.34 117 240 7% 0 6+ 58.6 -4.49 8 1.81 1081 283.23 35.40 118 240 7% 0 3 58.6 -5.6 8 1.42 1193 355.58 44.45 119 240 7% 0 6+ 58.6 -4.49 8 1.80 1027 428.41 53.55 240 7% 0 5+ 58.6 -3.34 8 1.90 1061 311.95 39.99 nd 119 (2 ) 327 Table D.5 (cont’d) rd 119 (3 ) 240 7% 0 4+ 58.6 -3.21 10 1.82 1074 170.93 17.09 120 180 6% 0 6+ 58.6 -4.49 8 1.72 970 87.02 10.88 124 180 5% 0 6+ 58.6 -4.49 8 1.73 990 73.08 9.13 125 240 7% 0 6+ 58.6 -4.49 8 1.68 1055 296.00 37.00 126 240 7% 0 6+ 58.6 -4.49 8 1.65 1015 131.11 16.39 127 220 6% 0 6+ 58.6 -4.49 8 1.81 1064 166.09 20.76 129 200 6% 0 6+ 58.6 -4.49 8 1.70 1058 150.00 18.75 131 220 5% 0 6+ 58.6 -4.49 8 1.75 1072 128.48 16.06 133 200 5% 0 6+ 58.6 -4.49 9.5 1.91 1058 137.59 14.48 134 240 7% 0 6+ 58.6 -4.49 8 1.70 1013 277.47 34.68 135 240 7% 0 3 58.6 -5.6 8 1.42 1228 182.86 22.86 328 Table D.5 (cont’d) 136 240 7% 0 3 58.6 -5.6 8 1.50 1220 208.84 26.10 139 260 5% 0 6+ 58.6 -4.49 8 1.59 1059 247.66 30.96 140 280 6% 0 6+ 58.6 -4.49 5.45 1.60 1072 196.59 36.07 141 280 5% 0 6+ 58.6 -4.49 8 1.62 1097 182.96 22.87 142 260 6% 0 6+ 58.6 -4.49 8 1.59 1066 270.34 33.79 143 (2 ) 280 5% 0 6+ 58.6 -4.49 8 1.63 1071 170.38 21.30 145 280 6% 0 6+ 58.6 -4.49 8 1.70 1074 234.16 29.27 240 7% 0 4+ 58.6 -3.21 12 1.57 1081 381.96 31.83 nd nd *S2 (2 ) * Sample for “side-surface growth” experiment. 329 BIBLIOGRAPHY 330 BIBLIOGRAPHY 1. 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